Guarantee cry of pain after case of Viagra's used up (6)
I believe the answer is:
avouch
'guarantee' is the definition.
(to avouch is to vouch for or guarantee)
'cry of pain after case of viagra's used up' is the wordplay.
'cry of pain' becomes 'ouch' (I've seen this in another clue).
'after' means one lot of letters go next to another.
'case of' suggests removing the centre (outsides of).
'used up' shows that the letters should be reversed in order (in a down clue, letters go up).
'viagra' with its middle removed is 'va'.
'va' back-to-front is 'av'.
'ouch' put after 'av' is 'AVOUCH'.
(Other definitions for avouch that I've seen before include "Assert; guarantee" , "Assert (archaic)" .)