Grumble increase's ultimately trivial (5)
I believe the answer is:
growl
'grumble' is the definition.
(I know that grumble can be written as growl)
'increase's ultimately trivial' is the wordplay.
'increase' becomes 'grow' (growing is a kind of increasing).
'ultimately' suggests the final letters.
The final letter of 'trivial' is 'l'.
'grow'+'l'='GROWL'
(Other definitions for growl that I've seen before include "Sound like hostile or angry dog" , "Menacing animal sound" , "surly utterance" , "Noise of an angry dog" , "Sound of hostile animal" .)