Lumber covers frame on Tower Bridge (8)
I believe the answer is:
straddle
'lumber' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for straddle that I've seen before include "Stretch legs across" , "Stand with legs either side of" , "Sit astride" , "jumping technique" , "Bestride" .)