Sweet, pixilated guys concealing a warm heart (6)
I believe the answer is:
sugary
'sweet' is the definition.
(similar in meaning)
'pixilated guys concealing a warm heart' is the wordplay.
'pixilated' is an anagram indicator.
'concealing' means one lot of letters goes inside another.
'heart' says to take the centre.
The central letters of 'warm' are 'ar'.
'guys' is an anagram of 'sugy'.
'sugy' placed around 'ar' is 'SUGARY'.
(Other definitions for sugary that I've seen before include "Over-sentimental" , "Sweet, perhaps too much so" , "Sweet, syrupy" , "Excessively sentimental" , "Excessively sweet or saccharine" .)