Slowly notice a soldier has nothing on (6)
I believe the answer is:
adagio
'slowly' is the definition.
(musical term)
'notice a soldier has nothing on' is the wordplay.
'notice' becomes 'ad' (short for advert).
'soldier' becomes 'GI' (GI can mean a US soldier).
'has' says to put letters next to each other.
'nothing on' becomes 'o' (I've seen this in other clues).
'ad'+'a'+'gi'+'o'='ADAGIO'
(Other definitions for adagio that I've seen before include "Slow movement (music)" , "Slow and graceful, musically" , "Slowly (mus.)" , "Leisurely, musically speaking" , "Leisurely tempo" .)