Strapping below tibia lost finally on the run (2,5)
I believe the answer is:
at large
'on the run' is the definition.
(I've seen this before)
'strapping below tibia lost finally' is the wordplay.
'strapping' becomes 'large' (I've seen this before).
'below' means one lot of letters go next to another (in a down clue, some letters go below others).
'finally' says to take the final letters.
The last letters of 'tibia lost' are 'at'.
'large' after 'at' is 'AT-LARGE'.
(Other definitions for at large that I've seen before include "'On the loose, escaped (2,5)'" , "On the run" , "Free to roam about, not confined" , "awaiting capture" , "a wild animal may roam so" .)