Alter altar for gay refrain (3-2)
I believe the answer is:
tra-la
'gay refrain' is the definition.
(I've seen this in another clue)
'alter altar' is the wordplay.
'alter' is an anagram indicator.
'altar' with letters rearranged gives 'TRA-LA'.
'for' is the link.
(Other definitions for tra-la that I've seen before include "Joyous refrain in song" , "Expression of joy or gaiety, musically" , "Ironic; expression of joy or gaiety" , "The flowers that bloom in the spring -- -- (Mikado)" , "Sound made when humming a song" .)