Calm partner holding ace (5)
I believe the answer is:
allay
'calm' is the definition.
(I've seen this before)
'partner holding ace' is the wordplay.
'partner' becomes 'ally' (an ally is an associate or supporter).
'holding' means one lot of letters goes inside another.
'ace' becomes 'a' (abbreviation).
'ally' placed around 'a' is 'ALLAY'.
(Other definitions for allay that I've seen before include "Lessen" , "Diminish or end (fears)" , "Lay to rest (fear)" , "Mollify or soothe, say fears" , "Diminish (fears/concern)" .)