Permit, new, kept by parasites, extremely credible (7)
I believe the answer is:
licence
'permit' is the definition.
(I know that permit can be written as licence)
'new kept by parasites extremely credible' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'kept by' indicates putting letters inside.
'parasites' becomes 'lice' (lice are parasitic insects).
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'credible' with its middle removed is 'ce'.
'lice'+'ce'='licece'
'n' inserted into 'licece' is 'LICENCE'.
(Other definitions for licence that I've seen before include "Freedom of action" , "Permit - freedom" , "Official permit" , "Official autorisation" , "Legal document giving official permission" .)