Pay that is horrible — there's mutinous activity about that (5,2)
I believe the answer is:
cough up
'pay that is horrible' is the definition.
'cough up' can be an answer for 'pay' (to cough up can mean to pay money). I am unsure of the rest of the definition.
'mutinous activity about that' is the wordplay.
'mutinous activity' becomes 'coup' (mutiny against a government or other authority).
'about' means one lot of letters goes inside another.
'that' becomes 'ugh' (I can't justify this - if you can you should believe this answer much more).
'coup' enclosing 'ugh' is 'COUGH UP'.
'there's' is the link.
(Other definitions for cough up that I've seen before include "Clear throat to part with money" , "Clear throat and pay what you owe" , "Pay out under duress" , "(Reluctantly) contribute" , "Pay money with some reluctance - clear throat" .)