Leaves, cycling A-to-B, calves, to begin with, firm (7)
I believe the answer is:
tobacco
'leaves' is the definition.
(I've seen this before)
'cycling a-to-b calves to begin with firm' is the wordplay.
'cycling' is an anagram indicator.
'to begin with' suggests taking the first letters.
'firm' becomes 'co' (a firm is a company).
The first letter of 'calves' is 'c'.
'a'+'to'+'b'='atob'
'atob' with letters rearranged gives 'toba'.
'toba'+'c'+'co'='TOBACCO'
(Other definitions for tobacco that I've seen before include "American export" , "Leaves to smoke" , "Addictive substance" , "Smoked narcotic plant" , "Eg, shag or snuff" .)