Weak father has to suffer (5)
I believe the answer is:
frail
'weak' is the definition.
(I know that weak can be written as frail)
'father has to suffer' is the wordplay.
'father' becomes 'fr' (abbreviation as in a priest's title).
'has' says to put letters next to each other.
'to suffer' becomes 'ail' (ailing is a kind of suffering).
'fr'+'ail'='FRAIL'
(Other definitions for frail that I've seen before include "Basket made of rushes" , "Decrepit" , "Weak and infirm" , "hobbling around?" , "Physically weak, delicate" .)