Carol fast approaching in SEAT (6,5)
I believe the answer is:
silent night
'carol' is the definition.
(I've seen this before)
'fast approaching in seat' is the wordplay.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'approaching' becomes 'nigh' (as in 'the end is nigh').
'in' means one lot of letters goes inside another.
'seat' becomes 'sit' (synonyms).
'lent'+'nigh'='lentnigh'
'lentnigh' going inside 'sit' is 'SILENT NIGHT'.
(Other definitions for silent night that I've seen before include "Christmas carol - hours of undisturbed sleep?" , "Blood sucking mammals" .)