Wander into quiet yard - it's fortified (6)
I believe the answer is:
sherry
'it's fortified' is the definition.
(as in fortified sherry)
'wander into quiet yard' is the wordplay.
'wander' becomes 'err'.
'into' indicates putting letters inside.
'quiet' becomes 'sh'.
'yard' becomes 'y'.
'sh'+'y'='shy'
'err' put within 'shy' is 'SHERRY'.
(Other definitions for sherry that I've seen before include "Alcoholic appetiser associated with Jerez" , "Jerez tipple" , "Strong sweet wine as aperitif" , "Pre-dinner aperitif as from Jerez" , "Oloroso or amontillado?" .)