At home, instruct one fellow to be smart (11)
I believe the answer is:
Here is my best explanation:
'be smart' is the definition.
('intelligent' can be similar in meaning to 'smart')
'at home instruct one fellow' is the subsidiary indication.
'at home' becomes 'in' ('in' can be similar in meaning to 'at home').
'instruct' becomes 'tell' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'fellow' becomes 'gent' ('gent' can be a synonym of 'fellow'**).
'to' is the link.
(Other definitions for intelligent that I've seen before include "Perspicacious", "quick on the uptake", "Reasoning; alert", "Able to reason; well-informed", "clever", "7 [CLEVER]", "Capable of reason", "Mentally acute", "displaying brainpower" and "Bright, quick of mind".)