Old church's financial backers lose pound sterling in trades (9)
I believe the answer is:
exchanges
'trades' is the definition.
'old church's financial backers lose pound sterling' is the wordplay.
'old' becomes 'ex'.
'church' becomes 'ch' (common abbreviation).
'financial backers' becomes 'angels'.
'lose' is a deletion indicator.
'pound sterling' becomes 'l' ( is an elaborate letter L).
'angels' with 'l' taken out is 'anges'.
'ex'+'ch'+'anges'='EXCHANGES'
'in' is the link.
(Other definitions for exchanges that I've seen before include "Thrusts and counter-thrusts" , "Swaps for sex change" , "tiffs" .)