Forces mounted on church in Greek island (7)
I believe the answer is:
coerces
'forces' is the definition.
(I know this)
'mounted on church in greek island' is the wordplay.
'mounted' says the letters should be written in reverse (in a down clue, letters mounted up).
'on' becomes 're' ('on' can be mean 'with reference to').
'church' becomes 'CE' (abbreviation for Church of England).
'in' indicates putting letters inside.
'greek island' becomes 'cos' (Cos is a Greek island).
're' reversed gives 'er'.
'er'+'ce'='erce'
'erce' inserted inside 'cos' is 'COERCES'.
(Other definitions for coerces that I've seen before include "Forces, pressures" , "Persuades by force" .)