A maxim Tom turns to (5)
I believe the answer is:
motto
'a maxim' is the definition.
(thesaurus)
'tom turns to' is the wordplay.
'turns' shows that the letters should be reversed in order.
'tom' in reverse letter order is 'mot'.
'mot'+'to'='MOTTO'
(Other definitions for motto that I've seen before include "Slogan encapsulating self-image" , "Message in cracker" , "It may be proverbial" , "Catchphrase" , "Saying in a cracker" .)