Cycle occasionally went in reverse – seriously (6)
I believe the answer is:
deeply
'seriously' is the definition.
(thesaurus)
'cycle occasionally went in reverse' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
'went' becomes 'peed' (both can mean urinated).
'in reverse' shows that the letters should be reversed in order.
The alternating letters of 'cycle' are 'yl'.
'yl'+'peed'='ylpeed'
'ylpeed' written backwards gives 'DEEPLY'.
(Other definitions for deeply that I've seen before include "With intense feeling" , "fathoms down?" , "With profundity" , "Intensely" , "Profoundly" .)