Book on cycling was clean (4)
I believe the answer is:
swab
'clean' is the definition.
(swabbing is a kind of cleaning)
'book on cycling was' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'on' says to put letters next to each other.
'cycling' is an anagram indicator.
'was' with letters rearranged gives 'swa'.
'b' put after 'swa' is 'SWAB'.
(Other definitions for swab that I've seen before include "Cleaning implement" , "Cotton wool mop" , "Wound-cleaner" , "Piece of fabric for medical use" , "Absorbent pad for cleansing or drying" .)