Shut up before classes unite (5,5)
I believe the answer is:
close ranks
'unite' is the definition.
(I've seen this before)
'shut up before classes' is the wordplay.
I cannot quite understand how this works, but
'shut' could be 'close' (synonyms) and 'close' is located in the answer.
'classes' could be 'ranks' (I've seen this in another clue) and 'ranks' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for close ranks that I've seen before include "make cabbies go out of business?" , "Unite in order to defend common interests" , "Unite as a defensive measure" , "to put taxi firms out of business?" , "Display solidarity" .)