Lower temperature necessary in the end above church altarpiece (7)
I believe the answer is:
diptych
'altarpiece' is the definition.
(thesaurus)
'lower temperature necessary in the end above church' is the wordplay.
'lower' becomes 'dip' (dipping is a kind of lowering).
'temperature' becomes 't' (abbreviation).
'in the end' says to take the final letters.
'above' means one lot of letters go next to another.
'church' becomes 'ch' (common abbreviation).
The final letter of 'necessary' is 'y'.
'dip'+'t'+'y'+'ch'='DIPTYCH'
(Other definitions for diptych that I've seen before include "Two-panel painting" , "Painting on two-hinged panels" , "Painting on two folding panels (7)" , "Picture with two panels" , "Pair of paintings on two hinged panels" .)