Tell a learner is relaxed (8)
I believe the answer is:
informal
'is relaxed' is the definition.
(informal can mean relaxed or casual)
'tell a learner' is the wordplay.
'tell' becomes 'inform' (I've seen this before).
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'inform'+'a'+'l'='INFORMAL'
(Other definitions for informal that I've seen before include "Casual, free and easy" , "Unofficial, relaxed" , "Casual and unceremonious" , "Cosy -- loose" , "Casual, not official" .)