Pay to spread manure - tree improved outside (10)
I believe the answer is:
remunerate
'pay' is the definition.
(I know that remunerate is a kind of pay)
'spread manure tree improved outside' is the wordplay.
'spread' indicates an anagram.
'improved' indicates an anagram.
'outside' means one lot of letters goes inside another (some letters go outside others).
'tree' is an anagram of 'rete'.
'manure' with letters rearranged gives 'munera'.
'munera' put within 'rete' is 'REMUNERATE'.
'to' is the link.
(Other definitions for remunerate that I've seen before include "Pay for service" , "reward" , "Make payment to" , "Pay (for work done)" .)