Firm has general exhibition on wheel (3)
I believe the answer is:
cog
'exhibition on wheel' is the definition.
'cog' can be an answer for 'wheel' (wheel with teeth in a machine). I'm unsure of the 'exhibition on' bit.
'firm has general' is the wordplay.
'firm' becomes 'co' (abbreviation for company).
'has' is a charade indicator (letters next to each other).
'general' becomes 'g' (this might be a standard abbreviation of which I'm unaware).
'co'+'g'='COG'
Can you help me to learn more?
(Other definitions for cog that I've seen before include "Projection on a toothed wheel" , "Junior official" , "One on a gearwheel" , "Tooth on gearwheel" , "One has teeth" .)