Standing by head of CID returning plain clothes (2,4)
I believe the answer is:
on call
'standing by' is the definition.
(someone on call is standing by to help)
'head of cid returning plain clothes' is the wordplay.
'head of' indicates taking the first letters.
'returning' says the letters should be written backwards.
'plain' becomes 'llano' (a grassy plain).
'clothes' means one lot of letters goes inside another (some letters clothe or cover others).
The initial letter of 'cid' is 'c'.
'llano' backwards is 'onall'.
'c' inserted into 'onall' is 'ON CALL'.
(Other definitions for on call that I've seen before include "available in emergencies?" , "Able to be contacted, like a GP" , "Contactable" , "Available if needed" , "ready to respond" .)