One gets tense before a transformed comic character, basically (2,6)

I believe the answer is:

at bottom

'basically' is the definition.
('at bottom' means basically or essentially)

I can't explain the remainder of the clue.

(Other definitions for at bottom that I've seen before include "Like the Titanic" , "Location of 27 28" , "Basically" .)

I've seen this clue in The Guardian.

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One gets tense before a trans­formed comic character, basically (2,6)