Notice rogue secreting second heart, say (8)
I believe the answer is:
emoticon
'heart say' is the definition.
(I know that is an emoticon)
'notice rogue secreting second' is the wordplay.
'rogue' indicates anagramming the letters (letters go rogue).
'secreting' is an insertion indicator (inserted letters are hidden or secreted).
'second' becomes 'mo' (both are short periods of time).
'notice' anagrammed gives 'eticon'.
'eticon' enclosing 'mo' is 'EMOTICON'.
(Other definitions for emoticon that I've seen before include "Smiley, eg" , "Screen symbol expressing feeling" , "Computer symbol showing mood" , "Symbol in email or text message to show joy etc" , "Representation of a facial expression in electronic messages" .)