Start working with gel (5)
I believe the answer is:
onset
'start' is the definition.
(both can mean the beginning of something)
'working with gel' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'with' says to put letters next to each other.
'gel' becomes 'set' (I've seen this in another clue).
'on'+'set'='ONSET'
(Other definitions for onset that I've seen before include "Inception" , "starting?" , "Cast stone in attack" , "Assault; beginning" , "Start, beginning" .)