He has a small life interest (12)
I believe the answer is:
entomologist
'he has a small life interest' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he has a small' is the wordplay.
I cannot quite see how this works, but
'he' could be 'tom' (I've seen this in another clue) and 'tom' is found in the answer.
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for entomologist that I've seen before include "One that finds 24 [INSECT]s" , "Person studying insects" , "Insect scientist" , "One who studies insects" , "Insect studier" .)