Mo was first to be amused (7)
I believe the answer is:
tickled
'be amused' is the definition.
(I've seen this in another clue)
'mo was first' is the wordplay.
'mo' becomes 'tick' (both can mean a short amount of time).
'was first' becomes 'led' (to lead is to go first).
'tick'+'led'='TICKLED'
'to' acts as a link.
(Other definitions for tickled that I've seen before include "Delighted" , "Pleased" , "Made to giggle" , "to be attracted to poacher" , "Amused" .)