Island captured by ordinary chap is worth a chortle (6)
I believe the answer is:
jocose
'worth a chortle' is the definition.
(jocose means humorous - chortling is laughing)
'island captured by ordinary chap' is the wordplay.
'island' becomes 'cos' (I've seen this before).
'captured by' indicates putting letters inside.
'ordinary chap' becomes 'Joe' (as in an 'average Joe').
'cos' placed within 'joe' is 'JOCOSE'.
'is' is the link.
(Other definitions for jocose that I've seen before include "Full of merriment" , "Humorous, waggish" , "Full of jokes" , "Facetious" , "Playful and joking" .)