Dismiss small rough unstable fellows (5,3)
I believe the answer is:
shrug off
'dismiss' is the definition.
(thesaurus)
'small rough unstable fellows' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'unstable' indicates an anagram.
'fellows' becomes 'ff'.
'rough' with letters rearranged gives 'hrugo'.
's'+'hrugo'+'ff'='SHRUG OFF'
(Other definitions for shrug off that I've seen before include "Rid oneself of" , "Get rid of - show indifference to" , "Dismiss (something) as unimportant" , "Brush aside, disregard the importance of" .)