Lying on back finally makes you ache (6)
I believe the answer is:
supine
'lying on back' is the definition.
(I've seen this before)
'finally makes you ache' is the wordplay.
'finally' suggests the final letters.
'ache' becomes 'pine' (synonyms).
The last letters of 'makes you' are 'su'.
'su'+'pine'='SUPINE'
(Other definitions for supine that I've seen before include "Inert; on one's back" , "Lying face upward" , "Laid-back" , "Lethargic" , "Unresisting" .)