Start to scrub grease stain (4)
I believe the answer is:
soil
'stain' is the definition.
(synonyms)
'start to scrub grease' is the wordplay.
'start to' suggests taking the first letters.
'grease' becomes 'oil' (I've seen this before).
The first letter of 'scrub' is 's'.
's'+'oil'='SOIL'
(Other definitions for soil that I've seen before include "refuse" , "Sully with earth" , "Top layer of the surface of the earth" , "Dirty - dirt" , "Earth; stain" .)