Palm hence to cross with silver? The reverse! (4)
I believe the answer is:
sago
'palm' is the definition.
(type of palm tree)
'hence to cross with silver? the reverse' is the wordplay.
'hence' becomes 'so'.
'to cross' means one lot of letters goes inside another.
'with silver?' becomes 'ga' (I can't explain this - if you can you should give a lot more credence to this answer).
'the reverse' says the letters should be written backwards.
'ga' written backwards gives 'ag'.
'so' going around 'ag' is 'SAGO'.
(Other definitions for sago that I've seen before include "Edible palm starch" , "Cereal, used in pudding" , "Type of milk pudding" , "Starchy cereal used in puddings" , "Starchy grain used in desserts" .)