Ask engineers to search (7)
I believe the answer is:
request
'ask' is the definition.
(I know that ask is a more specific form of the action request)
'engineers to search' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'to search' becomes 'quest' (quest is a kind of search).
're'+'quest'='REQUEST'
(Other definitions for request that I've seen before include "Appeal" , "Petition" , "Suit" , "Call for" , "bidding" .)