Extremely happy and rational over such mammals (6)
I believe the answer is:
hyenas
'mammals' is the definition.
(hyena is a kind of mammal)
'extremely happy and rational over' is the wordplay.
'extremely' means to remove the middle letters.
'and' means one lot of letters go next to another.
'rational' becomes 'sane' ('sane' can be similar in meaning to 'rational').
'over' says the letters should be written backwards.
'happy' with its middle removed is 'hy'.
'sane' backwards is 'enas'.
'hy'+'enas'='HYENAS'
'such' is the link.
(Other definitions for hyenas that I've seen before include "Flesh-eating dogs with laughter-like howl" , "Dog-like animals that sound as if they laugh" , "Dog-like animal of Africa" , "Animal whose cry sounds like wild laughter" , "Dog-like animals with laughter sounds" .)