Brisk stiffener? Get ice inside (9)
I believe the answer is:
energetic
'brisk' is the definition.
('energetic' can be similar in meaning to 'brisk')
'stiffener? get ice inside' is the wordplay.
'inside' says the answer is hidden in the clue.
'ENERGETIC' is hidden within 'stiffener get ice'.
(Other definitions for energetic that I've seen before include "Forceful, vigorous" , "Vigorous and dynamic" , "Lively, spirited" , "High-powered" , "Brisk" .)