Will includes it as auxiliary search (7)
I believe the answer is:
bequest
'will' is the definition.
(thesaurus)
'auxiliary search' is the wordplay.
'auxiliary' becomes 'be' (I am not sure about this - if you are sure you should believe this answer much more).
'search' becomes 'quest' (quest is a kind of search).
'be'+'quest'='BEQUEST'
'includes it as' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for bequest that I've seen before include "Endowment, legacy" , "One left in will" , "Something left in a will (7)" , "Gift left in will" , "Gift by will of personal property" .)