He said, 'I'm in!', but paused before acting (9)
I believe the answer is:
hesitated
'paused before acting' is the definition.
I can't tell whether this definition defines the answer.
'he said i'm in' is the wordplay.
'said' becomes 'stated' ('state' can be a synonym of 'say').
'i'm' becomes 'i' (I've seen this in other clues).
'in' indicates putting letters inside.
'stated' going around 'i' is 'sitated'.
'he'+'sitated'='HESITATED'
'but' is the link.
Can you help me to learn more?
(Other definitions for hesitated that I've seen before include "Thought twice" , "held back" , "Was not keen" , "Paused, wavered" , "was indecisive" .)