Former spouse joining performing group, to be precise (5)
I believe the answer is:
exact
'precise' is the definition.
(I know that precise can be written as exact)
'former spouse joining performing group' is the wordplay.
'former spouse' becomes 'ex' (an ex-spouse).
'joining' says to put letters next to each other.
'performing group' becomes 'act' (I've seen this before).
'ex'+'act'='EXACT'
'to be' is the link.
(Other definitions for exact that I've seen before include "19 [WREST]" , "far from improvised?" , "Strict" , "Precise - demand" , "Force out" .)