Place tart in cool (6)
I believe the answer is:
placid
'cool' is the definition.
(I've seen this before)
'place tart' is the wordplay.
'place' becomes 'pl' (abbreviation used in postal addresses).
'tart' becomes 'acid' (both can mean bitter-tasting).
'pl'+'acid'='PLACID'
'in' acts as a link.
(Other definitions for placid that I've seen before include "Quiet and still" , "or is it an American one?" , "Unexcitable" , "Still - US lake" , "Calm, unruffled" .)