Heather nursing English married couple's rodent (7)
I believe the answer is:
lemming
'rodent' is the definition.
(lemming is a kind of rodent)
'heather nursing english married couple's' is the wordplay.
'heather' becomes 'ling' ('ling' can be a synonym of 'heather').
'nursing' indicates putting letters inside.
'english' becomes 'e' (abbreviation).
'married' becomes 'm'.
'couple's' shows that letters should be duplicated (I've seen 'couple' mean this).
'm' doubled is 'mm'.
'e'+'mm'='emm'
'ling' going around 'emm' is 'LEMMING'.
(Other definitions for lemming that I've seen before include "Arctic rodent; one going blind to disaster" , "Supposedly suicidal animal" , "Northern rodent" , "Rodent renowned for mass migration" , "One misjudges landing" .)