Peer over line that's amusing (5)
I believe the answer is:
droll
'amusing' is the definition.
(thesaurus)
'peer over line' is the wordplay.
'peer' becomes 'lord' (I've seen this before).
'over' says the letters should be written in reverse.
'line' becomes 'l'.
'lord' back-to-front is 'drol'.
'drol'+'l'='DROLL'
'that's' acts as a link.
(Other definitions for droll that I've seen before include "Whimsical - amusing" , "Drily amusing" , "Clownish" , "Zany" , "Oddly amusing or comical" .)