Alf, straying out of line, will not succeed (4)
I believe the answer is:
fail
'not succeed' is the definition.
('fail' is the opposite to 'succeed')
'alf straying out of line will' is the wordplay.
'straying' indicates anagramming the letters.
'out of' is an insertion indicator (some letters on the outside of others).
'line will' becomes 'i' (I can't explain this - if you can you should give a lot more credence to this answer).
'alf' is an anagram of 'fal'.
'fal' enclosing 'i' is 'FAIL'.
Can you help me to learn more?
(Other definitions for fail that I've seen before include "Be unsuccessful, say in exam" , "Don't succeed" , "Run out; flop" , "Go wrong" , "Come to naught" .)