A game bird when worried, for instance, retreats (6)
I believe the answer is:
bridge
'a game' is the definition.
(card game)
'bird when worried for instance retreats' is the wordplay.
'when worried' indicates anagramming the letters (worry can mean to interfere with or disturb).
'for instance' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'retreats' says the letters should be written in reverse (I've seen 'retreating' mean this).
'eg' back-to-front is 'ge'.
'bird' is an anagram of 'brid'.
'brid'+'ge'='BRIDGE'
(Other definitions for bridge that I've seen before include "may involve suspension" , "Where four hands are needed" , "Card game for four players" , "Card game; structure" , "Overpass" .)