Lying that's done during function (6)
I believe the answer is:
supine
'lying' is the definition.
(I've seen this before)
'done during function' is the wordplay.
'done' becomes 'up' (both can mean finished or at an end).
'during' is an insertion indicator.
'function' becomes 'sine' (mathematical function).
'up' put within 'sine' is 'SUPINE'.
'that's' acts as a link.
(Other definitions for supine that I've seen before include "Lying face upwards, indolent" , "Slow in acting" , "Lying face upward" , "Lying on one's back - passive" , "Inert; on one's back" .)