Current supplier from Spain quit over stealing stereo, essentially (9)

'current supplier' is the definition.
The definition suggests a singular noun which matches the answer.

'spain quit over stealing stereo essentially' is the wordplay.
'spain' becomes 'e' (abbreviation for Espana).
'quit' becomes 'resign' (resigning is a kind of quiting).
'over' is a reversal indicator.
'stealing' means one lot of letters goes inside another.
'essentially' indicates the central letters.
The middle letters of 'stereo' are 'er'.
'resign' written backwards gives 'ngiser'.
'ngiser' enclosing 'er' is 'nergiser'.
'e'+'nergiser'='ENERGISER'

'from' is the link.

Can you help me to learn more?