Salad vegetable, mostly picked, over beside railway (6)
I believe the answer is:
celery
'salad vegetable' is the definition.
(I've seen this before)
'over beside railway' is the wordplay.
'over' becomes 'cele' (I can't justify this - if you can you should believe this answer much more).
'beside' means one lot of letters go next to another (I've seen this in other clues).
'railway' becomes 'ry' (abbreviation for railway).
'cele'+'ry'='CELERY'
'mostly picked' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for celery that I've seen before include "Stick vegetable" , "Plant with edible stalks" , "Salad item or cooked vegetable" , "Expedition" , "Perhaps sticks on dining table" .)