Size gained and tum's wobbling (9)
I believe the answer is:
magnitude
'size' is the definition.
(thesaurus)
'gained and tum's wobbling' is the wordplay.
'and' says to put letters next to each other (I've seen this in other clues).
'wobbling' is an anagram indicator.
'gained' after 'tum' is 'tumgained'.
'tumgained' is an anagram of 'MAGNITUDE'.
(Other definitions for magnitude that I've seen before include "(A star's) brightness" , "Largeness or size" , "Relative importance" , "Dimensions" , "Eating mud (anag.)" .)