Relation in Detroit arranged to be picked up (5)
I believe the answer is:
ratio
'relation' is the definition.
(ratio is a kind of relation)
'in detroit arranged to be picked up' is the wordplay.
'in' says the answer is hidden in the clue.
'to be picked up' is a reversal indicator.
'oitar' is hidden within 'detroit arranged'.
'oitar' in reverse letter order is 'RATIO'.
(Other definitions for ratio that I've seen before include "Proportion, relation" , "Proportional relationship" , "Relationship of numbers" , "Proportion between two numbers" , "Mathematical relationship" .)