Madden and mischievously pester a former associate at the start (10)
I believe the answer is:
exasperate
'madden' is the definition.
(thesaurus)
'mischievously pester a former associate at the start' is the wordplay.
'mischievously' indicates anagramming the letters (I've seen this in other clues).
'former' becomes 'ex'.
'associate' becomes 'a' (this might be a standard abbreviation of which I'm unaware).
'at the start' says to put letters next to each other.
'pester'+'a'='pestera'
'pestera' is an anagram of 'sperate'.
'ex'+'a'='exa'
'sperate' after 'exa' is 'EXASPERATE'.
'and' is the link.
(Other definitions for exasperate that I've seen before include "bug" , "Drive to distraction" , "Intensely irritate" , "See axe part (anag.)" , "Annoy" .)