Truly pretend to be posh to get partner (8)
I believe the answer is:
actually
'truly' is the definition.
(thesaurus)
'pretend to be posh to get partner' is the wordplay.
'pretend to be' becomes 'act' (I've seen this in another clue).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'to get' says to put letters next to each other.
'partner' becomes 'ally' (an ally is an associate or supporter).
'act'+'u'+'ally'='ACTUALLY'
(Other definitions for actually that I've seen before include "A lay cult may be existing in fact" , "In reality" , "Really existing" , "As a matter of fact" , "Really, in fact" .)