One act upset another not saying much (8)
I believe the answer is:
taciturn
'not saying much' is the definition.
(I've seen this before)
'one act upset another' is the wordplay.
'one' becomes 'i' (Roman numeral).
'upset' indicates anagramming the letters.
'another' becomes 'turn' (I can't explain this - if you can you should give a lot more credence to this answer).
'i'+'act'='iact'
'iact' is an anagram of 'taci'.
'taci'+'turn'='TACITURN'
Can you help me to learn more?
(Other definitions for taciturn that I've seen before include "Far from talkative" , "Quiet and uncommunicative" , "Disinclined towards conversation" , "Habitually uncommunicative, saying little" , "Tight-lipped" .)