An idea, however tense (7)
I believe the answer is:
thought
'an idea' is the definition.
('thought' can be a synonym of 'idea')
'however tense' is the wordplay.
'however' becomes 'though' ('though' can be a synonym of 'however').
'tense' becomes 't' (abbreviation used in many dictionaries).
'though'+'t'='THOUGHT'
(Other definitions for thought that I've seen before include "just a trifle" , "Idea coming to mind" , "Meditation" , "Fancy" , "Cogitation" .)