Number left in case of therapy ... (6)
I believe the answer is:
twenty
'number' is the definition.
(twenty is a kind of number)
'left in case of therapy' is the wordplay.
'left' becomes 'went' (I've seen this before**).
'in' is an insertion indicator.
'case of' says to hollow out the word (remove centre letters) (outsides of).
'therapy' with its middle removed is 'ty'.
'went' put within 'ty' is 'TWENTY'.
(Other definitions for twenty that I've seen before include "Cardinal" , "No. between 5 and 1 on dartboard" , "Score, XX" , "Hundredweights to a ton" , "No longer a teenager" .)