Write seriously, otherwise English soldier goes in (7)
I believe the answer is:
elegise
'write' is the definition.
'otherwise english soldier goes in' is the wordplay.
'otherwise' becomes 'else' (synonyms).
'english' becomes 'E' (abbreviation).
'soldier' becomes 'GI' (term for US soldier).
'goes in' indicates putting letters inside.
'e'+'gi'='egi'
'else' enclosing 'egi' is 'ELEGISE'.
'seriously' acts as a link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for elegise that I've seen before include "To write a funeral song" , "Write mournfully" .)