Sensible to control a learner (8)
I believe the answer is:
rational
'sensible' is the definition.
(associated in meaning)
'control a learner' is the wordplay.
'control' becomes 'ration' (to ration something is to control its supply).
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'ration'+'a'+'l'='RATIONAL'
'to' is the link.
(Other definitions for rational that I've seen before include "In accordance with logic" , "Thinking - not a liar (anag)" , "17 [INTELLIGENT]" , "Based on reason or logic" , "Consistent with logic" .)