Struggle to dissolve a tablet (6)
I believe the answer is:
battle
'struggle' is the definition.
(battling is a kind of struggling)
'dissolve a tablet' is the wordplay.
'dissolve' indicates an anagram.
'tablet' with letters rearranged gives 'BATTLE'.
'to' acts as a link.
(Other definitions for battle that I've seen before include "Struggle tenaciously" , "Lengthy struggle" , "Lengthy conflict" , "- - of 5 down" , "Fight between opponents" .)