They're wise, but only small time (5)
I believe the answer is:
sages
'they're wise but only' is the definition.
The definition suggests an adverb but the answer is not.
'small time' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'time' becomes 'ages' (I have seen 'Long time' mean 'ages' so perhaps 'time' could also mean 'ages').
's'+'ages'='SAGES'
Can you help me to learn more?
(Other definitions for sages that I've seen before include "Wise people" , "Pundits" , "enlightened people" , "Wise men" , "plants" .)