Lively people are full of them (5)
I believe the answer is:
beans
'lively people' is the definition.
The definition and answer can be both body parts as well as being plural nouns.
Maybe there's an association between them I don't understand?
'are full of them' is the wordplay.
I cannot quite see how this works, but
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is found in the answer.
The remaining letters 'bens' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for beans that I've seen before include "Edible kidney-shaped seeds" , "Haricots for instance" , "nuts" , "Pulses, tomato sauce and grilled bread" , "Haricots, e.g." .)