Old boy babbles about nursing a little silly infatuation (9)
I believe the answer is:
obsession
'infatuation' is the definition.
(thesaurus)
'old boy babbles about nursing a little silly' is the wordplay.
'old boy' becomes 'ob' (abbreviation).
'babbles' becomes 'noises' (I am not sure about this - if you are sure you should believe this answer much more).
'about' is a reversal indicator.
'nursing' is an insertion indicator.
'a little' indicates taking the first letters.
The initial letter of 'silly' is 's'.
'noises' written backwards gives 'sesion'.
'sesion' placed around 's' is 'session'.
'ob'+'session'='OBSESSION'
Can you help me to learn more?
(Other definitions for obsession that I've seen before include "Compulsion, fixation" , "Fan may display this" , "Phobia" , "Extreme interest" , "Unhealthy compulsion" .)