Primarily treacly, tiny, overly pretty (4)
I believe the answer is:
twee
'overly pretty' is the definition.
(I've seen this before)
'primarily treacly tiny' is the wordplay.
'primarily' says to take the initial letters.
'tiny' becomes 'wee' (I've seen this in another clue).
The initial letter of 'treacly' is 't'.
't'+'wee'='TWEE'
(Other definitions for twee that I've seen before include "Affectedly dainty or quaint" , "Sentimentally pretty" , "Affectedly sweet" , "Over-pretty, sentimental" , "Affectedly cute" .)