Injured a leg in the middle of steep descent (7)
I believe the answer is:
lineage
'descent' is the definition.
('lineage' can be a synonym of 'descent')
'injured a leg in the middle of steep' is the wordplay.
'injured' indicates an anagram.
'the middle of' indicates the central letters.
The centre of 'steep' is 'e'.
'a'+'leg'+'in'='alegin'
'alegin' with letters rearranged gives 'lineag'.
'lineag'+'e'='LINEAGE'
(Other definitions for lineage that I've seen before include "Descent (from an ancestor)" , "Direct descent" , "Stock" , "Ancestry, sequence of descendants" , "Line of descendants of an ancestor" .)