Boffin of inner depth (4)
I believe the answer is:
nerd
'boffin' is the definition.
(studious person)
'of inner depth' is the wordplay.
'of' indicates the answer is hidden within the clue.
'NERD' can be found hidden inside 'inner depth'.
(Other definitions for nerd that I've seen before include "One lacking social skills" , "Studious bore" , "Computer geek?" , "Boringly studious person" , "Intelligent but single-minded expert" .)