That was a silly mistake, on reflection, retaining clumsy cleaner (7)
I believe the answer is:
shampoo
'cleaner' is the definition.
(shampoo cleans hair)
'that was a silly mistake on reflection retaining clumsy' is the wordplay.
I cannot really understand how this works, but
'a' could be 'amp' (A is the symbol for the amp or ampere) and 'amp' is present in the answer.
The remaining letters 'shoo' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for shampoo that I've seen before include "Cleaning fluid" , "detergent" , "Hair-cleansing agent" , "people get set after it" , "hairdressing" .)