Saying harp is warped around old mop head (8)
I believe the answer is:
aphorism
'saying' is the definition.
(I know that aphorism is a type of axiom)
'harp is warped around old mop head' is the wordplay.
'warped' indicates anagramming the letters.
'around' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'head' says to take the initial letters.
The initial letter of 'mop' is 'm'.
'harp'+'is'='harpis'
'harpis' anagrammed gives 'aphris'.
'aphris' placed around 'o' is 'aphoris'.
'aphoris'+'m'='APHORISM'
(Other definitions for aphorism that I've seen before include "pithy statement" , "One could briefly say this" , "Saw" , "A short, pithy, instructive saying" , "Maxim, short proverb" .)