Concerned with uneven features of ignition? (4)
I believe the answer is:
into
'concerned with' is the definition.
(both can mean enthusiastic about)
'uneven features of ignition?' is the wordplay.
'uneven features of' means one should take alternating letters (only the letters in odd or un-even positions).
The alternate letters of 'ignition' are 'INTO'.
(Other definitions for into that I've seen before include "Knowledgeable about" , "digging" , "To a position within" , "Not I (anag.)" , "Expression of division" .)