Fail to win, receiving nothing, when relaxed (5)
I believe the answer is:
loose
'relaxed' is the definition.
(loose can mean flexible or unrestricted)
'fail to win receiving nothing' is the wordplay.
'fail to win' becomes 'lose' (synonyms).
'receiving' indicates putting letters inside.
'nothing' becomes 'O' (looks like zero - 0).
'lose' going around 'o' is 'LOOSE'.
'when' acts as a link.
(Other definitions for loose that I've seen before include "Lax - inaccurate - unbound" , "Unfixed" , "Slack; not exact" , "Water heater" , "Untie; dissolute" .)