After the start of the season, the side produces a source of power (5)
I believe the answer is:
steam
'a source of power' is the definition.
(I've seen this before)
'after the start of the season the side' is the wordplay.
'after' is a charade indicator (letters next to each other).
'the start of the season' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'the side' becomes 'team' (a side is a sporting team).
's' put next to 'team' is 'STEAM'.
'produces' is the link.
Can you help me to learn more?
(Other definitions for steam that I've seen before include "Kettle emission" , "Mates (anag)" , "fume" , "Watery vapour" , "Vapour from water at boiling point" .)