Compound charge applied to idiot (7)
I believe the answer is:
nitrate
'compound' is the definition.
(nitrate is a kind of compound)
'charge applied to idiot' is the wordplay.
'charge' becomes 'rate' (rate is a kind of charge**).
'applied to' means one lot of letters go next to another.
'idiot' becomes 'nit' (I've seen this before).
'rate' put after 'nit' is 'NITRATE'.
(Other definitions for nitrate that I've seen before include "Fertiliser chemical" , "Natural or synthetic fertiliser" , "Fertilizer" , "following 28 [SILVER]" , "salt" .)