Boss is given a letter to scrutinise (5)
I believe the answer is:
study
'scrutinise' is the definition.
(I've seen this before)
'boss is given a letter' is the wordplay.
'boss' becomes 'stud' (synonyms).
'is given a letter' becomes 'y' (I can't explain this - if you can you should believe this answer much more).
'stud'+'y'='STUDY'
'to' acts as a link.
(Other definitions for study that I've seen before include "Read to learn" , "Work-room" , "See 11" , "Contemplate" , "Scrutinise" .)