In cage, a gerbil's hungry (5)
I believe the answer is:
eager
'hungry' is the definition.
(I've seen this before)
'in cage a gerbil's' is the wordplay.
'in' says the answer is hidden in the clue.
'EAGER' is hidden within 'cage a gerbil'.
(Other definitions for eager that I've seen before include "Longing to see" , "Showing keen interest" , "passionate" , "Ardent" , "Zealous" .)