Camp doesn't start before survey's finished for current units (7)
I believe the answer is:
amperes
'current units' is the definition.
(ampere is a kind of current unit)
'camp doesn't start before survey's finished' is the wordplay.
'doesn't start' means to remove the first letter.
'before' becomes 'ere' (poetic term for 'before').
'survey's finished' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'camp' with its initial letter taken off is 'amp'.
'amp'+'ere'+'s'='AMPERES'
'for' acts as a link.
Can you help me to learn more?
(Another definition for amperes that I've seen is " Units of electrical current".)