Central tool on Iowa line (5)
I believe the answer is:
axial
'central' is the definition.
(thesaurus)
'tool on iowa line' is the wordplay.
'tool' becomes 'ax' (ax is a kind of tool).
'on' is a charade indicator (letters next to each other).
'iowa' becomes 'IA' (standard US state two-letter abbreviation).
'line' becomes 'l' (used when specifying particular lines from a poem).
'ax'+'ia'+'l'='AXIAL'
(Other definitions for axial that I've seen before include "Relating to lines of rotation" , "Going round with" , "Forming a line of rotation" , "Central Line?" , "Relating to a line of rotation" .)