A bitter end comes in the clutches of ale or porter (6)
I believe the answer is:
bearer
'porter' is the definition.
(I've seen this in another clue)
'a bitter end comes in the clutches of ale' is the wordplay.
'end' says to take the final letters.
'comes in' means one lot of letters goes inside another.
'the clutches of ale' becomes 'beer' (I can't justify this - if you can you should give a lot more credence to this answer).
The last letter of 'bitter' is 'r'.
'a'+'r'='ar'
'ar' going within 'beer' is 'BEARER'.
'or' acts as a link.
Can you help me to learn more?
(Other definitions for bearer that I've seen before include "Coffin carrier (one paid on demand?)" , "Carrier as of news" , "Carrier to whom cheque may be payable" , "He presents bill" , "One presenting cheque" .)