Check both sides of Stewart's Theorem (4)
I believe the answer is:
stem
'check' is the definition.
(thesaurus)
'both sides of stewart's theorem' is the wordplay.
'both sides of' suggests removing the centre (only the letters on the sides remain).
'theorem' becomes 'em' (I can't justify this - if you can you should believe this answer much more).
'stewart' with its centre taken out is 'st'.
'st'+'em'='STEM'
Can you help me to learn more?
(Other definitions for stem that I've seen before include "something growing?" , "Ski turn and plant stalk" , "Restrict - part of a plant" , "It supports a plant" , "Dam" .)