Solidly built deep sink (8)
I believe the answer is:
thickset
'solidly built' is the definition.
(I've seen this before)
'deep sink' is the wordplay.
'deep' becomes 'thick' (similar in meaning).
'sink' becomes 'set' (setting is a kind of sinking).
'thick'+'set'='THICKSET'
(Other definitions for thickset that I've seen before include "squat" , "Concentrated" , "Beefy" , "Compact" , "sturdy" .)