Expert has not returned straight away (2,4)
I believe the answer is:
at once
'straight away' is the definition.
'at once' can be an answer for 'away' (thesaurus). I'm not certain of the 'straight' bit.
'expert has not returned' is the wordplay.
'expert' becomes 'ace' (ace is a kind of expert).
'has' means one lot of letters goes inside another.
'returned' shows that the letters should be reversed in order.
'not' in reverse letter order is 'ton'.
'ace' placed around 'ton' is 'AT ONCE'.
Can you help me to learn more?
(Other definitions for at once that I've seen before include "This minute!" , "A no etc (anag)" , "from the kick-off" , "'Without delay (2,4)'" , "Right now, this instant" .)