Beat Iris and Ethel gutlessly on behind (10)
I believe the answer is:
flagellate
'beat' is the definition.
(I know that flagellate is a more specific form of the action flog)
'iris and ethel gutlessly on behind' is the wordplay.
'iris' becomes 'flag' (synonyms).
'and' is a charade indicator (letters next to each other).
'gutlessly' says to hollow out the word (remove centre letters).
'on behind' becomes 'late' (as in behind schedule. I am not sure about the 'on' bit.).
'ethel' with its middle removed is 'el'.
'flag'+'el'+'late'='FLAGELLATE'
(Other definitions for flagellate that I've seen before include "Whip" , "Scourge" .)