Gosh -- you can hear that army unit! (5)
I believe the answer is:
corps
'army unit' is the definition.
(corps is a kind of army unit)
'gosh you can hear' is the wordplay.
'gosh' becomes 'cor' (I've seen this before).
'you can hear' becomes 'ps' (I can't justify this - if you can you should give a lot more credence to this answer).
'cor'+'ps'='CORPS'
'that' is the link.
Can you help me to learn more?
(Other definitions for corps that I've seen before include "fighting group" , "Military body" , "Large army group" , "Military group" , "gang" .)