Nervous and pale losing heart (6)
I believe the answer is:
neural
'nervous' is the definition.
('neural' can be a synonym of 'nervous')
'pale losing heart' is the wordplay.
'pale' becomes 'neutral'.
'losing heart' says to hollow out the word (remove centre letters).
'neutral' with its middle taken out is 'NEURAL'.
'and' acts as a link.
(Other definitions for neural that I've seen before include "Relating to a nerve" , "Relating to the nervous system" , "Nervy" , "Relating to nerves" , "To do with nerve of unreal kind" .)