Lacking in strength (6)
I believe the answer is:
feeble
'lacking in strength' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for feeble that I've seen before include "Weak or ineffective" , "Wimpish" , "Weak - unconvincing" , "Weak, lacking strength" , "Very weak, without strength" .)