Sluggish time kept by Marseillaise composer's son (8)
I believe the answer is:
listless
'sluggish' is the definition.
(I've seen this in another clue)
'time kept by marseillaise composer's son' is the wordplay.
'time' becomes 't' (abbreviation).
'kept by' is an insertion indicator.
'marseillaise composer' becomes 'lisle' (I am not sure about this - if you are sure you should believe this answer much more).
'son' becomes 'ss' (I've seen this in other clues).
't' going into 'lisle' is 'listle'.
'listle'+'ss'='LISTLESS'
Can you help me to learn more?
(Other definitions for listless that I've seen before include "Uninterested" , "(unlike Ko-Ko)" , "without enthusiasm" , "Devoid of energy or interest" , "Devoid of spirit or energy" .)