Rat to watch on grain container, one flitting about (11,4)
I believe the answer is:
grasshopper mind
'one flitting about' is the definition.
'rat to watch on grain container' is the wordplay.
'rat' becomes 'grass' (slang term for an informer).
'to watch' becomes 'mind' (minding is a kind of watching).
'on' means one lot of letters go next to another.
'grain container' becomes 'hopper' (I've seen this before).
'mind' put after 'hopper' is 'hoppermind'.
'grass'+'hoppermind'='GRASSHOPPER MIND'
(Other definitions for grasshopper mind that I've seen before include "associated with something like cricket" , "one finds it hard to concentrate" , "having poor concentration" , "One shows little concentration" , "Reason for lack of concentration" .)