It's pitiful when dad gets the twitch (8)
I believe the answer is:
pathetic
'it's pitiful' is the definition.
(I know that pitiful can be written as pathetic)
'dad gets the twitch' is the wordplay.
'dad' becomes 'pa' (synonyms).
'gets' says to put letters next to each other.
'twitch' becomes 'tic' (tic is a kind of twitch).
'pa'+'the'+'tic'='PATHETIC'
'when' is the link.
(Other definitions for pathetic that I've seen before include "Tip cheat - truly sad" , "Sort of fallacy" , "Poignant or hopelessly inadequate" , "Wretched" , "Poor" .)