Stale perspiration, it seems, has long been soldierly (3,5)
I believe the answer is:
old sweat
'it seems has long been soldierly' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'stale perspiration' is the wordplay.
'stale' becomes 'old' (associated in meaning).
'perspiration' becomes 'sweat' (synonyms).
'old'+'sweat'='OLD SWEAT'
Can you help me to learn more?
(Other definitions for old sweat that I've seen before include "Very experienced chap" , "Veteran, one expected to 7" , "Veteran soldier" .)