Old church with mostly red pigment (5)
I believe the answer is:
ochre
'pigment' is the definition.
(ochre is a kind of pigment)
'old church with mostly red' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'church' becomes 'ch' (common abbreviation).
'with' means one lot of letters go next to another.
'mostly' means to remove the last letter.
'red' with its last letter removed is 're'.
'o'+'ch'+'re'='OCHRE'
(Other definitions for ochre that I've seen before include "Orangey colour" , "Pigment made from clay and iron oxide" , "Light yellow-brown colour" , "Yellow-orange pigment" , "yellowy-orange" .)