Mark's wrong to accept Young's tip (4)
I believe the answer is:
sign
'mark's' is the definition.
(synonyms)
'wrong to accept young's tip' is the wordplay.
'wrong' becomes 'sin' (wrong can mean a sin or crime).
'to accept' indicates putting letters inside.
'tip' indicates one should take the final letters.
The last letter of 'young' is 'g'.
'sin' placed around 'g' is 'SIGN'.
(Other definitions for sign that I've seen before include "Notice; symbol" , "Evidence" , "Autograph" , "part of the zodiac" , "Indicator" .)