Queen takes bishop - first obvious mistake (5)
I believe the answer is:
error
'mistake' is the definition.
('error' can be a synonym of 'mistake')
'queen takes bishop first obvious' is the wordplay.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'takes' means one lot of letters goes inside another.
'bishop' becomes 'rr' (abbreviation for Right Reverend).
'first' suggests taking the first letters.
The first letter of 'obvious' is 'o'.
'rr'+'o'='rro'
'er' going around 'rro' is 'ERROR'.
(Other definitions for error that I've seen before include "Slip, mistake" , "Howler" , "Bug" , "Solecism" , "Mistake from terrorist" .)