Having nothing on -- save time, ultimately (4)
I believe the answer is:
bare
'having nothing on' is the definition.
(having no clothes on)
'save time ultimately' is the wordplay.
'save' becomes 'bar' (I've seen this before).
'ultimately' says to take the final letters.
The final letter of 'time' is 'e'.
'bar'+'e'='BARE'
(Other definitions for bare that I've seen before include "Nude; uncover" , "Not clothed" , "State of deciduous tree in winter" , "Carried" , "Mere" .)