Clapham scored two in the first half to take his season tally to 13 in only 12 matches as Neath beat Ebbw Vale 22-10 in a hard-fought match at Eugene Cross Park. Former Ospreys backrower Hywel Jenkins came on as a replacement and secured the victory with a late try.
Neath have a game in hand and are three points clear at the top of the table, defending champions Newport lying second after a thrilling 22-20 win over Swansea at St Helen's.
It was the All Whites first home loss for 12 months, and came courtesy of a late try to impressive flanker Paul Williams.
Pontypridd demolished Llanharan 45-9, running in six tries at Sardis Road and limiting the visitors to three Wayne Jervis penalty goals, while Pontypool won the battle of the battlers when they beat Caerphilly 30-24.
Pooler trailed at halftime, but three quick goals from Jon Williams put them in front and they held on, despite a late try from Caerphilly lock Mark Workman.
Llanelli retain third place in the table with a solid 31-14 win at Newbridge, with outside-half Ceiron Thomas scoring a match-winning 21 points.
Aberavon scraped home 13-11 at Bedwas, while Cross Keys produced a gutsy 23-21 victory at Bridgend, coming back from being 6-10 down at halftime.
Welsh Premier Division results: Bedwas 11 Aberavon 13, Bridgend 21 Cross Keys 23, Ebbw Vale 10 Neath 22, Newbridge 14 Llanelli 31, Pontypool 30 Caerphilly 24, Pontypridd 45 Llanharan 9, Swansea 20 Newport 22. Postponed: Cardiff v Carmarthen Quins.