Count 150 bare pens on island (7)
I believe the answer is:
include
'count' is the definition.
(thesaurus)
'150 bare pens on island' is the wordplay.
'n50' becomes 'cl' (roman numerals).
'bare' becomes 'nude' (synonyms).
'pens' is an insertion indicator (to pen can mean to contain or imprison).
'on' means one lot of letters go next to another.
'island' becomes 'i'.
'cl' going into 'nude' is 'nclude'.
'nclude' put after 'i' is 'INCLUDE'.
(Other definitions for include that I've seen before include "Admit" , "Add as part of something else" , "Comprehend" , "Contain as part of group" , "Enclose or comprise as a part" .)