Nurse has to start trouble on purpose! (4)
I believe the answer is:
tend
'nurse' is the definition.
(tend can mean to nurse or look after)
'to start trouble on purpose' is the wordplay.
'to start' says to take the initial letters.
'on' means one lot of letters go next to another.
'purpose' becomes 'end' (I've seen this before).
The initial letter of 'trouble' is 't'.
't'+'end'='TEND'
'has' is the link.
(Other definitions for tend that I've seen before include "Look after, or take certain direction" , "Look after (a patient)" , "Care for, nurse" , "lean in a certain direction" , "Minister" .)