Set of three trickier extremities - pity about church (8)
I believe the answer is:
triptych
'set of three' is the definition.
(thesaurus)
'trickier extremities pity about church' is the wordplay.
'extremities' says to hollow out the word (remove centre letters).
'about' indicates anagramming the letters.
'church' becomes 'ch' (common abbreviation).
'trickier' with its middle taken out is 'tr'.
'pity' with letters rearranged gives 'ipty'.
'tr'+'ipty'+'ch'='TRIPTYCH'
(Other definitions for triptych that I've seen before include "Hit crypt (anag) - painting" , "Three-panel artwork" , "Three-part painting" , "Painting on three hinged panels" , "Picture with three panels" .)