Nowadays put lizard on your head as warning (10)

'warning' is the definition.
(synonyms)

'nowadays put lizard on your head' is the wordplay.
'nowadays' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'put' says to put letters next to each other.
'lizard' becomes 'monitor' (monitor is a kind of lizard).
'on' means one lot of letters go next to another.
'head' says to take the initial letters.
The initial letter of 'your' is 'y'.
'ad'+'monitor'+'y'='ADMONITORY'

'as' is the link.