Engineers search for petition (7)
I believe the answer is:
request
'for petition' is the definition.
(synonyms)
'engineers search' is the wordplay.
'engineers' becomes 're' (abbreviation for Royal Engineers).
'search' becomes 'quest' (quest is a kind of search).
're'+'quest'='REQUEST'
(Other definitions for request that I've seen before include "bidding" , "Suit" , "Appeal" , "Queer St (anag.)" , "Plea" .)