Bond star turns up, engaging lover once (5)
I believe the answer is:
nexus
'bond' is the definition.
(I've seen this before)
'star turns up engaging lover once' is the wordplay.
'star' becomes 'sun' (the Sun is a star).
'turns up' says the letters should be written in reverse.
'engaging' is an insertion indicator (inserted letters are engaged or involved in others).
'lover once' becomes 'ex' (an ex-lover).
'sun' backwards is 'nus'.
'nus' placed around 'ex' is 'NEXUS'.
(Other definitions for nexus that I've seen before include "Link - unsex (anag)" , "Connection; focal point" , "line" , "joined group" , "Bond, connected group" .)