Minute within fifteen years (5)
I believe the answer is:
teeny
'minute' is the definition.
(I've seen this before)
'within fifteen years' is the wordplay.
'within' says the answer is hidden in the clue.
'TEENY' can be found hidden inside 'fifteen years'.
(Other definitions for teeny that I've seen before include "Tiny" , "Minute" , "Very little relating" , "Extremely small" , "Minuscule" .)