Go both sides of the channel, overlapping briskly (7)
I believe the answer is:
allegro
'briskly' is the definition.
(thesaurus)
'go both sides of the channel overlapping' is the wordplay.
I cannot really understand how this works, but
'sides' could be 'll' (an abbreviation for left written twice) and 'll' is found within the answer.
'of' could be 'o' and 'o' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for allegro that I've seen before include "Musically fast and lively - legal or what?" , "Brisk movement or passage" , "Instruction to perform in lively fashion" , "'At a brisk speed, musically (7)'" , "quickly found in bars?" .)