Helping advanced student is reasonable (8)
I believe the answer is:
rational
'reasonable' is the definition.
('rational' is associated in meaning with 'reasonable')
'helping advanced student' is the wordplay.
'helping' becomes 'ration' (I've seen this before).
'advanced' becomes 'a'.
'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 "Sane; able to think" , "Clear-headed" , "Reasonable, logical" , "Endowed with reason" , "Logical, reasoned" .)