Sign of boredom? New approach backfired (4)
I believe the answer is:
yawn
'sign of boredom?' is the definition.
(I've seen this before)
'new approach backfired' is the wordplay.
'new' becomes 'n'.
'approach' becomes 'way'.
'backfired' is a reversal indicator.
'n'+'way'='nway'
'nway' reversed gives 'YAWN'.
(Other definitions for yawn that I've seen before include "Sleepy gape" , "Expression of tiredness" , "Intake of breath through a wide-open mouth" , "Show one is tired or bored" , "Boring event" .)