Old military leader is well (sadly not internally) (5)
I believe the answer is:
monty
'old military leader' is the definition.
(nickname of Field Marshal Bernard Montgomery)
'well sadly not internally' is the wordplay.
'well' becomes 'my' (both are expressions of surprise).
'sadly' indicates anagramming the letters.
'internally' is an insertion indicator.
'not' anagrammed gives 'ont'.
'my' going around 'ont' is 'MONTY'.
'is' is the link.
(Other definitions for monty that I've seen before include "Diminutive for Montgomery" , "Nickname of World War II general" , "The Full - - (film)" .)