Irregular sailor on choppy Roman lake (8)
I believe the answer is:
abnormal
'irregular' is the definition.
('abnormal' can be similar in meaning to 'irregular')
'sailor on choppy roman lake' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'on' is an insertion indicator.
'choppy' indicates an anagram.
'lake' becomes 'l'.
'roman' anagrammed gives 'norma'.
'ab' put into 'norma' is 'abnorma'.
'abnorma'+'l'='ABNORMAL'
(Other definitions for abnormal that I've seen before include "Unorthodox" , "Not typical or usual" , "Deviant or unnatural" , "Rum" , "Different to what is ordinary or expected" .)