The girl will have power to get resin for varnish (7)
I believe the answer is:
shellac
'for varnish' is the definition.
(shellac varnish is a kind of varnish)
'the girl will have power' is the wordplay.
'the girl will' becomes 'she'll'.
'have' says to put letters next to each other.
'power' becomes 'AC' (AC is an example).
'shell'+'ac'='SHELLAC'
'to get resin' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for shellac that I've seen before include "Resin in flakes, used to making varnish" , "(it's in old records)" , "Defeat" , "Resin from trees" , "Sort of resin" .)