Slow, but so tricky even at the outset (6)
I believe the answer is:
obtuse
'slow' is the definition.
(I know that slow can be written as obtuse)
'but so tricky even at the outset' is the wordplay.
'tricky' indicates an anagram.
'at the outset' suggests taking the first letters (the outset is the beginning).
The first letter of 'even' is 'e'.
'but'+'so'='butso'
'butso' with letters rearranged gives 'obtus'.
'obtus'+'e'='OBTUSE'
(Other definitions for obtuse that I've seen before include "Annoyingly insensitive" , "Blunt, dull-witted" , "To be thus blunt" , "Thick, blunt" , "Insensitive - more than a right angle" .)