Cow being not as fast, doesn't start (5)
I believe the answer is:
lower
'cow' is the definition.
(I've seen this in other clues)
'not as fast doesn't start' is the wordplay.
'not as fast' becomes 'slower' ('slow' can be the opposite of 'fast' so perhaps 'slower' is reasonable for 'not as fast').
'doesn't start' means to remove the first letter.
'slower' with its first letter removed is 'LOWER'.
'being' acts as a link.
(Other definitions for lower that I've seen before include "Depress" , "Threaten" , "disgrace" , "Bring down" , "where are trousers in relation?" .)