Coward, lacking bounce, is getting on in years (2,6,7)
I believe the answer is:
no spring chicken
'getting on in years' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'coward lacking bounce' is the wordplay.
I cannot quite see how this works, but
'coward' could be 'chicken' (similar in meaning) and 'chicken' is present in the answer.
'lacking' could be 'no' (eg lacking money means with no money) and 'no' is located in the answer.
'bounce' could be 'spring' (synonyms) and 'spring' is present in the answer.
No letters remain.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Other definitions for no spring chicken that I've seen before include "Mature" , "One who's seasoned" , "not young any more" , "unseasoned meat, might you say?" , "Veteran" .)