Language of love undermining Heather (5)
I believe the answer is:
lingo
'language' is the definition.
(lingo is a kind of language)
'love undermining heather' is the wordplay.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'undermining' says to put letters next to each other.
'heather' becomes 'ling' ('ling' can be a synonym of 'heather').
'o' after 'ling' is 'LINGO'.
'of' acts as a link.
(Other definitions for lingo that I've seen before include "Language (informally)" , "Johnny Foreigner speaks one" , "Parlance" , "Log in (anag.)" , "Specialised vocabulary" .)