Poet joining regiment shortly (7)
I believe the answer is:
sassoon
'poet' is the definition.
(WW1 poet Siegfried Sassoon)
'joining regiment shortly' is the wordplay.
'joining' says to put letters next to each other.
'regiment' becomes 'SAS' (Special Air Service).
'shortly' becomes 'soon' (synonyms).
'sas' next to 'soon' is 'SASSOON'.
(Other definitions for sassoon that I've seen before include "Poet condemning war" , "Siegfried --, poet and writer" , "Siegfried ---, war poet" , "Siegfried --, English writer" , "Siegfried --, English poet and novelist" .)