Staying-power of Santa, flying across motorway! (7)
I believe the answer is:
stamina
'staying-power' is the definition.
(I've seen this before)
'santa flying across motorway' is the wordplay.
'flying' indicates an anagram (letters fly around into new positions).
'across' means one lot of letters goes inside another.
'motorway' becomes 'MI' (resembles M1, English motorway).
'santa' anagrammed gives 'stana'.
'stana' placed around 'mi' is 'STAMINA'.
'of' is the link.
(Other definitions for stamina that I've seen before include "Ability to maintain effort" , "perseverance" , "Ability to keep going" , "having more to call on" , "energy to go on" .)