Clear up broken axle-pin (7)
I believe the answer is:
explain
'clear up' is the definition.
(to clear up is to explain or clarify)
'broken axle-pin' is the wordplay.
'broken' indicates an anagram.
'axle'+'pin'='axlepin'
'axlepin' is an anagram of 'EXPLAIN'.
(Other definitions for explain that I've seen before include "Describe" , "Justify" , "offer clarification" , "Defend" , "Elucidate, make clear" .)