It's obvious when a learner's in trouble (5)
I believe the answer is:
plain
'it's obvious' is the definition.
(similar in meaning)
'a learner's in trouble' is the wordplay.
'a learner' becomes 'l' (as in a learner driver with L-plates on their car).
'in' indicates putting letters inside.
'trouble' becomes 'pain' (synonyms).
'l' placed into 'pain' is 'PLAIN'.
'when' acts as a link.
(Other definitions for plain that I've seen before include "Simple - flatland" , "Geographical feature" , "Simple, ordinary" , "no oil painting features" , "Simple or apparent" .)