Dad joining Church — step forward (4)
I believe the answer is:
pace
'step forward' is the definition.
(to pace is to step or walk around)
'dad joining church' is the wordplay.
'dad' becomes 'pa' (pa can mean someone's father).
'joining' is a charade indicator (letters next to each other).
'church' becomes 'ce' (abbreviation for Church of England).
'pa'+'ce'='PACE'
(Other definitions for pace that I've seen before include "Rate of progress" , "Lick" , "Clip" , "Part of step" , "a certain distance" .)