Sink back - water at last to slip away (7)
I believe the answer is:
relapse
'sink back' is the definition.
(thesaurus)
'water at last to slip away' is the wordplay.
'at last' suggests the final letters.
'to slip away' becomes 'elapse' (synonyms).
The last letter of 'water' is 'r'.
'r'+'elapse'='RELAPSE'
(Other definitions for relapse that I've seen before include "Fall back into old ways" , "Fall back like leapers" , "Return to sickness" , "Get into bad habits again" , "Fall back into sickness" .)