Priest carrying English carol-singer, we hear, who’s very light (13)
I believe the answer is:
featherweight
'light' is the definition.
(thesaurus)
'priest carrying english carol-singer' is the wordplay.
'priest' becomes 'father' (father is the title for a priest).
'carrying' means one lot of letters goes inside another.
'english' becomes 'E' (abbreviation as in OED).
'carol-singer' becomes 'weight' (I can't explain this - if you can you should believe this answer much more).
'father' placed around 'e' is 'feather'.
'feather'+'weight'='FEATHERWEIGHT'
'we hear who's very' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for featherweight that I've seen before include "Light boxer" , "Boxing category" , "one might be counted out?" , "Nobody" , "Boxer weighing up to 57 kg" .)