We use solver to solve (9)
I believe the answer is:
ourselves
'we' is the definition.
(I've seen this before)
'use solver to solve' is the wordplay.
'to solve' indicates an anagram (I've seen 'solved' mean this (the anagram must be solved)).
'use'+'solver'='usesolver'
'usesolver' is an anagram of 'OURSELVES'.
(Other definitions for ourselves that I've seen before include "See 16" , "Us, reflexively" , "We or us personally" , "Emphatically us" .)