Fine-tune first of devices used in a fair (6)
I believe the answer is:
adjust
'fine-tune' is the definition.
(I've seen this before)
'first of devices used in a fair' is the wordplay.
'first of' suggests taking the first letters.
'used in' means one lot of letters goes inside another.
'fair' becomes 'just' ('just' can be a synonym of 'fair'**).
The first letter of 'devices' is 'd'.
'a'+'just'='ajust'
'd' going into 'ajust' is 'ADJUST'.
(Other definitions for adjust that I've seen before include "Calibrate or regulate" , "suit" , "Bring small change" , "Modify - tweak" , "Adapt (oneself)" .)