Enlist who is set to work in support for the opposition? (7,7)

I believe the answer is:

hostile witness

'support for the opposition?' is the definition.

'enlist who is set to work' is the wordplay.
'to work' indicates an anagram.
'enlist'+'who'+'is'+'set'='enlistwhoisset'
'enlistwhoisset' with letters rearranged gives 'HOSTILE WITNESS'.

'in' acts as a link.

(Another definition for hostile witness that I've seen is " One testifying unexpectedly".)

I've seen this clue in The Independent.

Want a hint initially instead of a full solution? Install my app