Heart, say: tick in it once erratic (8)
I believe the answer is:
emoticon
'heart say' is the definition.
(I know that heart is a emoticon)
'tick in it once erratic' is the wordplay.
'tick' becomes 'mo' (both can mean a very short amount of time).
'in' is an insertion indicator.
'erratic' indicates an anagram.
'it'+'once'='itonce'
'itonce' is an anagram of 'eticon'.
'mo' inserted into 'eticon' is 'EMOTICON'.
(Other definitions for emoticon that I've seen before include "Group of characters online" , "A combination of characters" , "Graphic way of conveying feeling in e-mails" , "Wordless expression" , "Screen symbol expressing feeling" .)