Bag flimsy, splitting in the end (5)
I believe the answer is:
thing
'bag' is the definition.
(thesaurus)
'flimsy splitting in the end' is the wordplay.
'flimsy' becomes 'thin' (similar in meaning).
'in the end' indicates one should take the final letters.
The last letter of 'splitting' is 'g'.
'thin'+'g'='THING'
(Other definitions for thing that I've seen before include "Object, artefact" , "A creature" , "An unnamed object or item" , "Object; gadget" , "Affair - matter" .)