Engineers start to conduct church survey (5)
I believe the answer is:
recce
'survey' is the definition.
(I've seen this before)
'engineers start to conduct church' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'start to' suggests taking the first letters.
'church' becomes 'ce' (abbreviation for Church of England).
The initial letter of 'conduct' is 'c'.
're'+'c'+'ce'='RECCE'
(Other definitions for recce that I've seen before include "Military survey" , "Informal appraisal" , "Survey (sl.)" , "Scouting expedition (colloq.)" , "Mil. survey" .)