Go and nut ordinary couple of fellows (4,3)
I believe the answer is:
head off
'go' is the definition.
(I've seen this before)
'nut ordinary couple of fellows' is the wordplay.
'nut' becomes 'head' (nut can mean one's head).
'ordinary' becomes 'O' (abbreviation e.g. O-Levels).
'couple of' indicates the doubling of some letters.
'fellows' becomes 'f' (this might be a standard abbreviation which I don't know about).
'f' doubled is 'ff'.
'head'+'o'+'ff'='HEAD OFF'
'and' is the link.
Can you help me to learn more?
(Other definitions for head off that I've seen before include "Stop" , "Deflect from a path" , "Turn away" , "Intercept - at the pass?" , "prevent" .)