Father in agony (4)
I believe the answer is:
pain
'agony' is the definition.
(I've seen this in another clue)
'father in' is the wordplay.
'father' becomes 'pa' (pa can mean dad or father).
'pa'+'in'='PAIN'
(Other definitions for pain that I've seen before include "Physical discomfort" , "Suffering, ache" , "great discomfort" , "Mental distress" , "Tiresome figure" .)