Unpleasant new head coming to stay, perhaps (5)
I believe the answer is:
nasty
'unpleasant' is the definition.
('nasty' can be a synonym of 'unpleasant')
'new head coming to stay perhaps' is the wordplay.
'head' indicates taking the first letters.
'coming to' is a charade indicator (letters next to each other).
'perhaps' is an anagram indicator.
The initial letter of 'new' is 'n'.
'stay' anagrammed gives 'asty'.
'n'+'asty'='NASTY'
(Other definitions for nasty that I've seen before include "Highly repugnant" , "Spiteful and unkind" , "Horrid, repugnant" , "Horrible" , "Not nice" .)