Search thoroughly to find first bit of custard in tart (5)
I believe the answer is:
scour
'search thoroughly' is the definition.
(I've seen this before)
'first bit of custard in tart' is the wordplay.
'first bit of custard' becomes 'c' (1st letter of 'custard').
'in' indicates putting letters inside.
'tart' becomes 'sour' (similar in meaning).
'c' put inside 'sour' is 'SCOUR'.
'to find' is the link.
(Other definitions for scour that I've seen before include "Clean by hard rubbing" , "Comb" , "Turn upside down" , "Scrub hard" , "Clean - scout" .)