Market put up inside old church (8)
I believe the answer is:
exchange
'market' is the definition.
(I've seen this before)
'put up inside old church' is the wordplay.
'put up' becomes 'hang' (as in hanging a picture).
'inside' is an insertion indicator.
'old' becomes 'ex' (prefix meaning former or onetime).
'church' becomes 'CE' (Church of England).
'ex'+'ce'='exce'
'hang' inserted inside 'exce' is 'EXCHANGE'.
(Other definitions for exchange that I've seen before include "Stock trading site" , "Fight -- angel chews box (anag)" , "Short conversation or argument" , "Barter; switch" , "Give one thing and get another back" .)