Bar complicated computing craze for market trading (9)
I believe the answer is:
arbitrage
'market trading' is the definition.
(I've seen this before)
'bar complicated computing craze' is the wordplay.
'complicated' is an anagram indicator (a complex or confusing ordering of the letters).
'computing' becomes 'it' (IT is an abbreviation for Information Technology).
'craze' becomes 'rage' (synonyms).
'bar' anagrammed gives 'arb'.
'arb'+'it'+'rage'='ARBITRAGE'
'for' acts as a link.
(Other definitions for arbitrage that I've seen before include "Making a turn" , "Financial juggling" , "Market trading" , "Market dealer's activity" .)