University drill's about being kept inside - hard, but sheltered (9)
I believe the answer is:
harboured
'sheltered' is the definition.
(harbouring is a kind of sheltering)
'university drill's about being kept inside hard' is the wordplay.
'university' becomes 'u' (abbreviation for university).
'drill' becomes 'bore' (synonyms).
'about' means one lot of letters goes inside another.
'being kept inside' indicates putting letters inside.
'u' placed inside 'bore' is 'boure'.
'boure' inserted within 'hard' is 'HARBOURED'.
'but' acts as a link.
(Other definitions for harboured that I've seen before include "Sheltered" , "found shelter" , "concealed" .)