Tell how the axle pin broke (7)
I believe the answer is:
explain
'tell' is the definition.
(explaining is a kind of telling)
'the axle pin broke' is the wordplay.
'broke' indicates anagramming the letters.
'axle'+'pin'='axlepin'
'axlepin' anagrammed gives 'EXPLAIN'.
'how' is the link.
(Other definitions for explain that I've seen before include "Elucidate - define" , "Defend" , "Make understandable" , "Justify" , "Give reasons for" .)