Weak fellow fronting bar (5)
I believe the answer is:
frail
'weak' is the definition.
('frail' can be similar in meaning to 'weak')
'fellow fronting bar' is the wordplay.
'fronting' says to take the initial letters (I've seen 'front' mean this).
'bar' becomes 'rail' (rail is a kind of bar**).
The initial letter of 'fellow' is 'f'.
'f'+'rail'='FRAIL'
(Other definitions for frail that I've seen before include "Delicate, fragile" , "Physically weak, delicate" , "Feeble - basket" , "old and weak" , "Physically feeble" .)