Informative idea can ultimately spell out trouble (11)
I believe the answer is:
educational
'informative' is the definition.
(similar in meaning)
'idea can ultimately spell out trouble' is the wordplay.
'ultimately' suggests the final letters.
'trouble' indicates anagramming the letters.
The last letter of 'spell' is 'l'.
'idea'+'can'+'l'+'out'='ideacanlout'
'ideacanlout' anagrammed gives 'EDUCATIONAL'.
(Other definitions for educational that I've seen before include "Relating to teaching" , "Didactic" , "A docile aunt (anag) - instructive" , "Providing knowledge" , "as an aid to learning" .)