Weak father's trouble (5)
I believe the answer is:
frail
'weak' is the definition.
('frail' can be similar in meaning to 'weak')
'father's trouble' is the wordplay.
'father' becomes 'fr'.
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble'**).
'fr'+'ail'='FRAIL'
(Other definitions for frail that I've seen before include "Easily shattered" , "Physically weak, delicate" , "Feeble - basket" , "Puny" , "Weak, feeble" .)