Bid of 500 to protect damaged reef (7)
I believe the answer is:
offered
'bid' is the definition.
(I've seen this in another clue)
'of 500 to protect damaged reef' is the wordplay.
'500' becomes 'D' (Roman numeral for 500).
'to protect' indicates putting letters inside (inserted letters are held or protected).
'damaged' indicates anagramming the letters.
'of'+'d'='ofd'
'reef' anagrammed gives 'fere'.
'ofd' enclosing 'fere' is 'OFFERED'.
(Other definitions for offered that I've seen before include "Presented for acceptance or rejection" , "Made a bid" , "Supplied" , "Expressed readiness (to do something)" , "Made available" .)