A weight, carrying coals to Newcastle? On the contrary (5)
I believe the answer is:
stone
'a weight' is the definition.
(stone is a kind of weight)
'newcastle? on the contrary' is the wordplay.
'newcastle?' becomes 'enos' (I can't explain this - if you can you should believe this answer much more).
'on' is an insertion indicator (as in clothing \'on\' a person).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'contrary' shows that the letters should be reversed in order.
'enos' going around 't' is 'enots'.
'enots' back-to-front is 'STONE'.
'carrying coals to' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
(Other definitions for stone that I've seen before include "that one will 26d at 8a [FLING at HIGHLAND GAMES]" , "Gem; Staffs. town" , "Piece of rock - weight" , "See 5" , "Endocarp" .)