Proceed clumsily — you fall behind, reversing at speed (7)
I believe the answer is:
galumph
'proceed clumsily' is the definition.
(I know this)
'you fall behind reversing at speed' is the wordplay.
'you' becomes 'u' (commonly used in online communication).
'fall behind' becomes 'lag' (synonyms).
'reversing' says the letters should be written in reverse.
'at' is a charade indicator (letters next to each other).
'speed' becomes 'mph' (abbreviation for miles per hour).
'u'+'lag'='ulag'
'ulag' in reverse letter order is 'galu'.
'galu'+'mph'='GALUMPH'
(Other definitions for galumph that I've seen before include "Leap about clumsily" , "move inelegantly" , "proceed awkwardly" , "Bound so excitedly" , "Move clumsily, ponderously (colloq.)" .)