Suffers to become sallow! (6)
I believe the answer is:
allows
'suffers' is the definition.
The answer and definition can be both related to thinking as well as being verbs in their -s form.
Maybe there's a link between them I don't understand?
'become sallow' is the wordplay.
'become' indicates anagramming the letters.
'sallow' with letters rearranged gives 'ALLOWS'.
'to' is the link.
Can you help me to learn more?
(Other definitions for allows that I've seen before include "Tolerates" , "Gives permission" , "Permits, lets" , "Permits to do something" , "Permits or agrees" .)