A lease secured by the gymnast (7)
I believe the answer is:
athlete
'gymnast' is the definition.
(I've seen this before)
'a lease secured by the' is the wordplay.
'lease' becomes 'let' ('let' can be a synonym of 'lease').
'secured by' is an insertion indicator (I've seen this in other clues).
'let' placed within 'the' is 'thlete'.
'a'+'thlete'='ATHLETE'
(Other definitions for athlete that I've seen before include "Olympic participant" , "Bolt perhaps" , "Player" , "The late one is very fit, oddly" , "eg Dean Macey" .)