Helping a student is reasonable (8)
I believe the answer is:
rational
'reasonable' is the definition.
('rational' is associated in meaning with 'reasonable')
'helping a student' is the wordplay.
'helping' becomes 'ration' (I've seen this before).
'student' becomes 'l' (as in L-plates for learner drivers).
'ration'+'a'+'l'='RATIONAL'
'is' is the link.
(Other definitions for rational that I've seen before include "Consistent with reason" , "Clearly thinking" , "Able to reason" , "Using reason" , "Based on logic or reasoning" .)