Helpful alibi fence cooked up (10)
I believe the answer is:
beneficial
'helpful' is the definition.
(I've seen this before)
'alibi fence cooked up' is the wordplay.
'cooked up' indicates an anagram.
'alibi'+'fence'='alibifence'
'alibifence' is an anagram of 'BENEFICIAL'.
(Other definitions for beneficial that I've seen before include "profitable" , "Favourable, advantageous" , "helpful" .)