Long time at the detention centre (5)
I believe the answer is:
yearn
'long' is the definition.
(synonyms)
'time at the detention centre' is the wordplay.
'time' becomes 'year' (I've seen this in other clues).
'at' is a charade indicator (letters next to each other).
'centre' means to look at the middle letters.
The central letter of 'detention' is 'n'.
'year'+'n'='YEARN'
(Other definitions for yearn that I've seen before include "Pant - pine" , "Feel desire" , "Have an intense feeling of longing" , "Have a great longing" , "Curdle" .)