Boss has gin cocktail after end of day’s perusing (8)
I believe the answer is:
studying
'perusing' is the definition.
(synonyms)
'boss has gin cocktail after end of day's' is the wordplay.
'boss' becomes 'stud' (synonyms).
'has' is a charade indicator (letters next to each other).
'cocktail' indicates anagramming the letters.
'after' is a charade indicator (letters next to each other).
'end of' indicates one should take the final letters.
The final letter of 'day' is 'y'.
'gin' with letters rearranged gives 'ing'.
'ing' after 'y' is 'ying'.
'stud' next to 'ying' is 'STUDYING'.
(Other definitions for studying that I've seen before include "In education" , "Acquiring knowledge" , "Doing what a student should" , "Reading carefully with the intention to remember" .)