Take cat first of all into school (7)
I believe the answer is:
stomach
'take' is the definition.
(thesaurus)
'cat first of all into school' is the wordplay.
'cat' becomes 'tom' (a tom is a male cat).
'first of' indicates taking the first letters.
'into' indicates putting letters inside.
'school' becomes 'sch'.
The initial letter of 'all' is 'a'.
'tom'+'a'='toma'
'toma' inserted inside 'sch' is 'STOMACH'.
(Other definitions for stomach that I've seen before include "Appetite for food" , "Bear - belly" , "stand" , "Take" , "Tolerate; appetite" .)