Sound cheerful while going round the short way (7)
I believe the answer is:
whistle
'sound cheerful' is the definition.
(I've seen this before)
'while going round the short way' is the wordplay.
'going round' indicates putting letters inside (some letters go around others).
'the short way' becomes 'st' (abbreviation for street; 'short' suggests a short form).
'while' going around 'st' is 'WHISTLE'.
(Other definitions for whistle that I've seen before include "What ref blows" , "Call vainly for" , "Shrill piping sound" , "Instrument; suit (slang)" , "Make cheerful sound" .)