Urge to cross island, small one (4)
I believe the answer is:
eigg
'small one' is the definition.
I know nothing about this answer so I can't judge whether this works.
'urge to cross island' is the wordplay.
'urge' becomes 'egg' (as in egging someone on).
'to cross' is an insertion indicator (some letters cross or go round others).
'island' becomes 'i' (geographical abbreviation).
'egg' placed around 'i' is 'EIGG'.
Can you help me to learn more?
(Other definitions for eigg that I've seen before include "Scottish island" , "Isle of Inner Hebrides" , "land in sea" , "Hebridean island" .)