Football player beginning to sob? Crybaby! (7)
I believe the answer is:
sweeper
'football player' is the definition.
(I know that sweeper is a type of centre-back)
'beginning to sob? crybaby' is the wordplay.
'beginning to' indicates taking the first letters.
'crybaby' becomes 'weeper' (I've seen this in another clue).
The initial letter of 'sob' is 's'.
's'+'weeper'='SWEEPER'
(Other definitions for sweeper that I've seen before include "Soccer defender" , "cleaner" , "Soccer player" , "'Football defender, carries a brush (7)'" , "One using broom" .)