Abundant crop, if spreading, smothers large island (8)
I believe the answer is:
prolific
'abundant' is the definition.
(thesaurus)
'crop if spreading smothers large island' is the wordplay.
'spreading' is an anagram indicator.
'smothers' indicates putting letters inside (I've seen this in other clues).
'large' becomes 'L' (eg in clothes sizes).
'island' becomes 'i' (geographical abbreviation).
'crop'+'if'='cropif'
'cropif' is an anagram of 'profic'.
'l'+'i'='li'
'profic' going around 'li' is 'PROLIFIC'.
(Other definitions for prolific that I've seen before include "Abundant in fruit etc" , "Plentiful" , "Very fertile, productive" , "Abundant in growth" , "Producing much" .)