Regularly turn nasty and difficult (6)
I believe the answer is:
tricky
'difficult' is the definition.
(similar in meaning)
'regularly turn nasty' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'nasty' becomes 'icky' (I've seen this before).
The alternating letters of 'turn' are 'tr'.
'tr'+'icky'='TRICKY'
'and' is the link.
(Other definitions for tricky that I've seen before include "Ticklish" , "Deceitful, crafty" , "Crafty or difficult" , "Difficult; sly" , "Crafty, foxy, slippery" .)