On a chart, they indicate islands with wild boars (7)
I believe the answer is:
isobars
'on a chart they indicate' is the definition.
The definition suggests an adverb but the answer is not.
'islands with wild boars' is the wordplay.
'islands' becomes 'is' (i is an abbreviation for island).
'with' indicates putting letters inside.
'wild' indicates an anagram.
'boars' with letters rearranged gives 'sobar'.
'is' going around 'sobar' is 'ISOBARS'.
Can you help me to learn more?
(Other definitions for isobars that I've seen before include "Map lines of equal pressure" , "15 is strong when they are close" , "can show what pressure folk are under" , "What may be featured on map" , "Lines linking places of equal barometric pressure" .)