Hellcat transported by boy set off (6,2)
I believe the answer is:
switch on
'set off' is the definition.
(I've seen this before)
'hellcat transported by boy' is the wordplay.
'hellcat' becomes 'witch' (I've seen this in another clue).
'transported by' is an insertion indicator (some letters carried or transported by others).
'boy' becomes 'son' (synonyms).
'witch' inserted into 'son' is 'SWITCH ON'.
(Other definitions for switch on that I've seen before include "Engage an electrical device" , "get to work" , "fire" , "Make contact with" , "Operate" .)