The small, older one on the inside is in the army (7)
I believe the answer is:
soldier
'the army' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'the small older one on the inside' is the wordplay.
'the small' becomes 's' (abbreviation - e.g. clothes size).
'one' becomes 'i' (Roman numeral).
'on the inside' is an insertion indicator.
'older' enclosing 'i' is 'oldier'.
's'+'oldier'='SOLDIER'
'is in' acts as a link.
Can you help me to learn more?
(Other definitions for soldier that I've seen before include "One enlisted" , "Member of the military" , "piece of bread" , "trained killer" , "Person serving in army" .)