Smooth touch in case of serious pollutants (3,6)

'pollutants' is the definition.

'smooth touch in case of serious' is the wordplay.
'smooth' becomes 'oil' (to oil or grease something is to smooth it).
'touch' becomes 'lick' (lick is a kind of touch).
'in' indicates putting letters inside.
'case of' means to remove the middle letters (outsides of).
'serious' with its middle taken out is 'ss'.
'lick' put into 'ss' is 'slicks'.
'oil'+'slicks'='OIL SLICKS'