Give a hand when son is on time (6)
I believe the answer is:
assist
'give a hand' is the definition.
'when son is on time' is the wordplay.
'when' becomes 'as'.
'son' becomes 's'.
'on' says to put letters next to each other.
'time' becomes 't'.
'as'+'s'+'is'+'t'='ASSIST'
(Other definitions for assist that I've seen before include "Work as a subordinate" , "Aid, abet" , "provide backing" , "Support" , "Understand by instinct" .)