Erratic when having miles to run (7)
I believe the answer is:
aimless
'erratic' is the definition.
(thesaurus)
'when having miles to run' is the wordplay.
'when' becomes 'as'.
'having' means one lot of letters goes inside another.
'to run' indicates an anagram.
'miles' anagrammed gives 'imles'.
'as' placed around 'imles' is 'AIMLESS'.
(Other definitions for aimless that I've seen before include "With no plan or programme" , "Melissa (anag) -- unmotivated" , "Without purpose or target" , "at a loose end" , "Lacking ambition or target" .)