Tree or fruit mainly seen across west of Yemen (6)
I believe the answer is:
banyan
'tree' is the definition.
(I know that banyan is a type of fig tree)
'fruit mainly seen across west of yemen' is the wordplay.
'fruit' becomes 'banana' (type of fruit).
'mainly' means to remove the last letter (virtually or mainly the word, but not all of it).
'seen across' indicates putting letters inside.
'west of' suggests taking the first letters (most west - leftmost - letter).
The initial letter of 'yemen' is 'y'.
'banana' with its final letter taken off is 'banan'.
'banan' placed around 'y' is 'BANYAN'.
'or' is the link.
(Other definitions for banyan that I've seen before include "Fig tree with aerial roots" , "Multi-trunked Indian tree" , "Any ban for an Indian tree" , "one that does give a fig" , "plant" .)