Ineffectual Sue on manoeuvres (2,3)
I believe the answer is:
no use
'ineffectual' is the definition.
(I've seen this before)
'sue on manoeuvres' is the wordplay.
'manoeuvres' indicates an anagram.
'sue'+'on'='sueon'
'sueon' anagrammed gives 'NO USE'.
(Other definitions for no use that I've seen before include "won't help?" , "Futile" , "Pointless" , "Ineffectual" .)