As desperate last resort, try to grab things to put into drink (5,2,6)
I believe the answer is:
grasp at straws
'as desperate last resort' is the definition.
(to grasp at straws is to try things desperately)
'try to grab things to put into drink' is the wordplay.
I cannot really understand how this works, but
'try to grab' could be 'graspat' (I've seen this in another clue) and 'graspat' is present in the answer.
The remaining letters 'straws' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for grasp at straws that I've seen before include "Resort to desperate measures" , "Desperately try inadequate solutions" , "Take any chance" , "Resort to inadequate remedy in desperation" , "Seek desperate means" .)