Line worker put around church window (6)
I believe the answer is:
lancet
'window' is the definition.
(lancet window is a kind of window)
'line worker put around church' is the wordplay.
'line' becomes 'l'.
'worker' becomes 'ant' (worker is a type of ant).
'put around' indicates putting letters inside.
'church' becomes 'ce' (abbreviation for Church of England).
'ant' going around 'ce' is 'ancet'.
'l'+'ancet'='LANCET'
(Other definitions for lancet that I've seen before include "provided cuttings" , "Surgeon's tool" , "A surgeon's two-edged knife" , "Surgeon's instrument and journal" , "Sharp instrument" .)