Salad ingredient offered by engineer on the French railway (6)
I believe the answer is:
celery
'salad ingredient' is the definition.
(I've seen this in another clue)
'engineer on the french railway' is the wordplay.
'engineer' becomes 'ce' (abbreviation for civil engineer).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'the french' becomes 'le' ('the' in French).
'railway' becomes 'ry' (abbreviation for railway).
'ce'+'le'+'ry'='CELERY'
'offered by' acts as a link.
(Other definitions for celery that I've seen before include "Salad item or cooked vegetable" , "Perhaps sticks on dining table" , "Leafstalks eaten as a vegetable" , "Vegetable in sticks" , "Plant with edible stems" .)