Lacking physical strength (4)
'lacking physical strength' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for feeble that I've seen before include "Weak or Fragile" , "rather weak" , "Wimpish" , "Lacking physical strength" , "lacking resolve" .)
(Other definitions for weak that I've seen before include "Feeble, powerless" , "Lacking strength" , "Wanting in physical strength" , "Easily led" , "Spent" .)