Certify fluid cleans step evenly (7)
I believe the answer is:
license
'certify' is the definition.
(I know that certify can be written as license)
'fluid cleans step evenly' is the wordplay.
'evenly' means one should take alternating letters (the letters in even-numbered positions).
'fluid'+'cleans'+'step'='fluidcleansstep'
The alternate letters of 'fluidcleansstep' are 'LICENSE'.
(Other definitions for license that I've seen before include "Entitle" , "Silence (anag.)" , "Authorise (something)" , "Authorise to sell in silence" , "Grant official permit to sell or trade" .)