Jack allowed to inspire son to be most virtuous (7)
I believe the answer is:
noblest
'be most virtuous' is the definition.
'jack allowed to inspire son' is the wordplay.
'jack' becomes 'nob' (I've seen this before**).
'allowed' becomes 'let' (I've seen this before).
'to inspire' indicates putting letters inside (breathe in).
'son' becomes 's'.
'let' placed around 's' is 'lest'.
'nob'+'lest'='NOBLEST'
'to' is the link.
(Other definitions for noblest that I've seen before include "Most illustrious" , "Most magnificent" , "Most dignified" , "Most distinguished" , "most generous" .)