Bring up troops to north, crossing island (7)
I believe the answer is:
mention
'bring up' is the definition.
(bring up can mean to mention in conversation)
'troops to north crossing island' is the wordplay.
'troops' becomes 'men' (I've seen this in another clue).
'north' becomes 'N' (synonyms).
'crossing' means one lot of letters goes inside another.
'island' becomes 'i'.
'to'+'n'='ton'
'ton' enclosing 'i' is 'tion'.
'men'+'tion'='MENTION'
(Other definitions for mention that I've seen before include "Cite; quote" , "Allude to" , "state" , "Briefly refer to (something)" , "Make reference to" .)