On-the-spot coffee, say - no use, a disaster (13)
I believe the answer is:
instantaneous
'on-the-spot' is the definition.
(I've seen this before)
'coffee say no use a disaster' is the wordplay.
'coffee say' becomes 'instant' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'disaster' indicates anagramming the letters.
'no'+'use'+'a'='nousea'
'nousea' with letters rearranged gives 'aneous'.
'instant'+'aneous'='INSTANTANEOUS'
Can you help me to learn more?
(Other definitions for instantaneous that I've seen before include "Immediate" , "On-the-spot" , "Direct" , "Momentary" , "Done in a moment" .)