Mum's budgerigars, finally flying, cheep not so much (10)
I believe the answer is:
speechless
'mum's' is the definition.
(thesaurus)
'budgerigars finally flying cheep not so much' is the wordplay.
'finally' says to take the final letters.
'flying' is an anagram indicator.
'not so much' becomes 'less' (I've seen this before).
The last letter of 'budgerigars' is 's'.
'cheep' is an anagram of 'peech'.
's'+'peech'+'less'='SPEECHLESS'
(Other definitions for speechless that I've seen before include "Dumbfounded" , "being forced to improvise?" , "Lost for words" , "Unable to say anything" , "Dumbstruck" .)