Subsequently swathing top of head in soap (6)
I believe the answer is:
lather
'soap' is the definition.
(synonyms)
'subsequently swathing top of head' is the wordplay.
'subsequently' becomes 'later' (subsequently means later on).
'swathing' indicates putting letters inside (swathe can mean to wrap up or cover).
'top of' suggests taking the first letters.
The first letter of 'head' is 'h'.
'later' going around 'h' is 'LATHER'.
'in' acts as a link.
(Other definitions for lather that I've seen before include "Frothy suds" , "Shaving or soap foam" , "Foamy soap as for shaving" , "starting to run in a sweat" , "Mass of froth" .)