Terribly tense, rapid walker (10)
I believe the answer is:
pedestrian
'walker' is the definition.
(I know that walker can be written as pedestrian)
'terribly tense rapid' is the wordplay.
'terribly' is an anagram indicator.
'tense'+'rapid'='tenserapid'
'tenserapid' anagrammed gives 'PEDESTRIAN'.
(Other definitions for pedestrian that I've seen before include "Dull; walker" , "lacking inspiration" , "One walks around" , "Hiker" , "Walker; prosaic" .)