Alliance fit to secure goal beyond doubt (14)
I believe the answer is:
unquestionable
'beyond doubt' is the definition.
('unquestionable' can be similar in meaning to 'beyond doubt')
'alliance fit to secure goal' is the wordplay.
'alliance' becomes 'un' (UN is an example).
'fit' becomes 'able' ('able' can be similar in meaning to 'fit').
'to' means one lot of letters go next to another (I've seen this in other clues).
'secure goal' becomes 'question' (I can't explain this - if you can you should give a lot more credence to this answer).
'able' put after 'question' is 'questionable'.
'un'+'questionable'='UNQUESTIONABLE'
(Another definition for unquestionable that I've seen is " beyond doubt".)