Solitary man guarding front of goal going into extra time (6)
I believe the answer is:
longer
'going into extra time' is the definition.
'solitary man guarding front of goal' is the wordplay.
'solitary man' becomes 'loner' (**).
'guarding' is an insertion indicator.
'front of' indicates taking the first letters.
The first letter of 'goal' is 'g'.
'loner' enclosing 'g' is 'LONGER'.
(Other definitions for longer that I've seen before include "Less short" , "Measuring greater distance end to end" , "Of greater length" , "More extended" , "Of greater duration" .)