Father has time to knock (3)
I believe the answer is:
pat
'to knock' is the definition.
(pat can mean to knock or tap)
'father has time' is the wordplay.
'father' becomes 'pa' (pa can mean dad or father).
'has' says to put letters next to each other.
'time' becomes 't' (abbreviation).
'pa'+'t'='PAT'
(Other definitions for pat that I've seen before include "Affectionate gesture - postman" , "Glib; touch affectionately" , "Gentle stroke of the hand with fluent repetition" , "Touch; name" , "well-rehearsed" .)