Trial of old fairy getting ridiculous (12)
I believe the answer is:
experimental
'trial' is the definition.
(synonyms)
'old fairy getting ridiculous' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'fairy' becomes 'peri' (Persian mythological being).
'getting' means one lot of letters go next to another.
'ridiculous' becomes 'mental' (mental can informally mean ridiculous or bizarre).
'ex'+'peri'+'mental'='EXPERIMENTAL'
'of' is the link.
(Other definitions for experimental that I've seen before include "Relating to laboratory investigation" , "Exploratory" , "Based on untested ideas" , "Empirical" , "as pilot" .)