Reserve mare trashed Noah's boat (7)
I believe the answer is:
earmark
'reserve' is the definition.
(I know that reserve can be written as earmark)
'mare trashed noah's boat' is the wordplay.
'trashed' is an anagram indicator.
'noah's boat' becomes 'ark' (I've seen this in another clue).
'mare' anagrammed gives 'earm'.
'earm'+'ark'='EARMARK'
(Other definitions for earmark that I've seen before include "Set aside for later" , "Set aside, designate" , "'Reserve, set aside (7)'" , "Designate funds for a particular purpose" , "reserve for future use" .)