Little fellow isn't feeble (5)
I believe the answer is:
faint
'feeble' is the definition.
(I've seen this before)
'little fellow isn't' is the wordplay.
'little' indicates taking the first letters.
'isn't' becomes 'aint' (I've seen this before).
The first letter of 'fellow' is 'f'.
'f'+'aint'='FAINT'
(Other definitions for faint that I've seen before include "Swoon, pass out" , "dull" , "woozy" , "Barely perceptible" , "Lacking clarity" .)