Returning computer nuisance, it's said, in military operations (8)
I believe the answer is:
campaign
'military operations' is the definition.
(I've seen this before)
'returning computer nuisance it's said in' is the wordplay.
'returning' says the letters should be written in reverse.
'computer' becomes 'mac' (I've seen this before).
'nuisance' becomes 'pain' (synonyms).
'it's said' becomes 'g' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'mac' reversed gives 'cam'.
'pain' placed around 'g' is 'paign'.
'cam'+'paign'='CAMPAIGN'
Can you help me to learn more?
(Other definitions for campaign that I've seen before include "electioneering" , "Series of actions toward a particular end" , "offensive" , "Crusase, drive" , "Series of operations, military or advertising" .)