He can't have been in uniform all that long (5)
I believe the answer is:
cadet
'have been in uniform all that long' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'he can't' is the wordplay.
'he' becomes 'Cad'.
'can't' becomes 'et' (I can't explain this - if you can you should give a lot more credence to this answer).
'cad'+'et'='CADET'
Can you help me to learn more?
(Other definitions for cadet that I've seen before include "officer of the future" , "not the heir" , "Police force trainee" , "Officer in training" , "young soldier" .)