Delicate father to sicken (5)
I believe the answer is:
frail
'delicate' is the definition.
(I know that delicate can be written as frail)
'father to sicken' is the wordplay.
'father' becomes 'fr' (abbreviation as in a priest's title).
'to sicken' becomes 'ail' (I've seen this before).
'fr'+'ail'='FRAIL'
(Other definitions for frail that I've seen before include "rush" , "Struggling to get about" , "Decrepit" , "Physically feeble" , "Delicate, fragile" .)