Hand in hand start (5)
I believe the answer is:
first
'start' is the definition.
(synonyms)
'hand in hand' is the wordplay.
'hand' becomes 'R' (abbreviation for right, as in the right-hand side).
'in' means one lot of letters goes inside another.
'hand' becomes 'fist' (hand curled into a fist).
'r' placed into 'fist' is 'FIRST'.
(Other definitions for first that I've seen before include "Initial" , "at the outset" , "Rifts (anag)" , "Highest degree" , "Rifts of supreme sort" .)