Able to run, following target of chase? (3)
I believe the answer is:
fit
'able to run' is the definition.
'following target of chase?' is the wordplay.
'following' becomes 'f' (used when citing page numbers etc).
'target of chase?' becomes 'it' (as in children's games like it or tag).
'f'+'it'='FIT'
(Other definitions for fit that I've seen before include "Able-bodied" , "Appropriate -- not 21" , "In good physical condition, healthy" , "Paroxysm" , "Adequate" .)