Stalk US baseball players from the south (4)
I believe the answer is:
stem
'stalk' is the definition.
(synonyms)
'us baseball players from the south' is the wordplay.
'us baseball players' becomes 'Mets' (New York Mets).
'from the south' is a reversal indicator (in a down clue, letters go up).
'mets' reversed gives 'STEM'.
(Other definitions for stem that I've seen before include "Upward part of a plant" , "Main stalk of a plant" , "Block" , "Peduncle" , "Restrict - part of a plant" .)