Read endlessly as boss (4)
I believe the answer is:
stud
'boss' is the definition.
(synonyms)
'read endlessly' is the wordplay.
'read' becomes 'study' (synonyms).
'endlessly' means to remove the last letter.
'study' with its final letter removed is 'STUD'.
'as' is the link.
(Other definitions for stud that I've seen before include "Macho man" , "old wooden post" , "Animal breeding farm" , "Projecting boss" , "Small jewellery item" .)