Crack makes previous partner bold (6)
I believe the answer is:
expert
'crack' is the definition.
(thesaurus)
'previous partner bold' is the wordplay.
'previous partner' becomes 'ex' (I've seen this in other clues).
'bold' becomes 'pert' (I've seen this in another clue).
'ex'+'pert'='EXPERT'
'makes' is the link.
(Other definitions for expert that I've seen before include "Master" , "Person with special knowledge or ability" , "The person who knows" , "Skilled workman or specialist" , "Masterly" .)