Disdainful having set up extremely nervous suspect (6)
I believe the answer is:
sniffy
'disdainful' is the definition.
(synonyms)
'set up extremely nervous suspect' is the wordplay.
'set up' is a reversal indicator (in a down clue, letters go up).
'extremely' suggests removing the centre (letters on the extremes of the word are used).
'suspect' becomes 'iffy' (both can mean dubious).
'nervous' with its middle taken out is 'ns'.
'ns' back-to-front is 'sn'.
'sn'+'iffy'='SNIFFY'
'having' acts as a link.
(Other definitions for sniffy that I've seen before include "Supercilious; snobby" , "Disdainful" , "Scornful, contemptuous" .)