Mark was first to be amused (7)
I believe the answer is:
tickled
'be amused' is the definition.
(thesaurus)
'mark was first' is the wordplay.
'mark' becomes 'tick' (I've seen this before**).
'was first' becomes 'led'.
'tick'+'led'='TICKLED'
'to' acts as a link.
(Other definitions for tickled that I've seen before include "Delighted" , "to be attracted to poacher" , "Pleased" , "Amused" , "Made to giggle" .)