A haircut to excite endless mirth? (4)
I believe the answer is:
trim
'a haircut' is the definition.
(I've seen this in another clue)
'excite endless mirth?' is the wordplay.
'excite' is an anagram indicator.
'endless' means to remove the last letter.
'mirth' with its final letter taken away is 'mirt'.
'mirt' with letters rearranged gives 'TRIM'.
'to' acts as a link.
(Other definitions for trim that I've seen before include "Light hair-cut in Meath" , "Well-kept - tidy" , "Cut neatly; light haircut" , "Neat and ticy in Meath" , "Cut, prune; decorate" .)