Unfriendly, when a clown shows up (5)
I believe the answer is:
aloof
'unfriendly' is the definition.
(I've seen this before)
'a clown shows up' is the wordplay.
'clown' becomes 'fool' (fool is a kind of clown).
'shows up' is a reversal indicator.
'fool' in reverse letter order is 'loof'.
'a'+'loof'='ALOOF'
'when' is the link.
(Other definitions for aloof that I've seen before include "Disinterested" , "apart from all else" , "Unpleasantly cold" , "Distant or stuck-up" , "Blood deficiency" .)