Endlessly examine poker (4)
I believe the answer is:
stud
'poker' is the definition.
(I know that stud is a type of poker)
'endlessly examine' is the wordplay.
'endlessly' means to remove the last letter.
'examine' becomes 'study' (synonyms).
'study' with its final letter removed is 'STUD'.
(Other definitions for stud that I've seen before include "Breeding stable" , "Collar-fastener" , "Gripper under a boot" , "Collar fastener - form of poker" , "Collar fastener or horse farm" .)