Immoderate posh officer joins gunners (5)
I believe the answer is:
ultra
'immoderate' is the definition.
(similar in meaning)
'posh officer joins gunners' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'officer' becomes 'lt' (I've seen this in other clues).
'joins' says to put letters next to each other.
'gunners' becomes 'ra' (Royal Artillery).
'u'+'lt'+'ra'='ULTRA'
(Other definitions for ultra that I've seen before include "(As prefix) beyond or extreme" , "Extreme (eg in fashion or politics)" , "Violet's predecessor" , "Secret intelligence" , "lacking moderation" .)