Old habit of city gents: get naked under cover of trees (10)

'old habit of city gents' is the definition.
(design on old-fashioned clothes)

'get naked under cover of trees' is the wordplay.
'get naked' becomes 'strip' (strip off clothing).
'under cover of' means one lot of letters goes inside another (inserted letters get covered up by others).
'trees' becomes 'pines' (pine is a kind of tree).
'strip' put within 'pines' is 'PINSTRIPES'.