Strain to see bird on church (6)
I believe the answer is:
wrench
'strain' is the definition.
(thesaurus)
'bird on church' is the wordplay.
'bird' becomes 'wren' (wren is a kind of bird).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'church' becomes 'ch' (abbreviation on maps etc).
'wren'+'ch'='WRENCH'
'to see' is the link.
(Other definitions for wrench that I've seen before include "Jerky pulling movement" , "Violent twist; tool" , "Sprain; spanner" , "A spanner, or a muscular twist" , "Spanner-like tool" .)