Crossword Genius

It's free from tree farm or shopping centre? (4)

Ross

I believe the answer is:

mart

I'm a little stuck... Click here to teach me more about this clue!

'it's free from tree farm' is the definition.
I can't tell whether this defines the answer.

'or shopping centre?' is the wordplay.
'or' becomes 'mar' (I can't explain this - if you can you should believe this answer much more).
'shopping' becomes 'retailtherapy' (I've seen this before).
'centre?' means to look at the middle letters (I've seen 'centre' mean this).
The central letter of 'retailtherapy' is 't'.
'mar'+'t'='MART'

Can you help me to learn more?

(Other definitions for mart that I've seen before include "Trading centre" , "US shop; market" , "Place" , "Take tram to market" .)

I've seen this clue in the Irish Times (Crosaire) and the Irish Times.
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