Fed up with following when running out of breath (6)
I believe the answer is:
puffed
'out of breath' is the definition.
(I've seen this before)
'fed up with following when running' is the wordplay.
'with' means one lot of letters go next to another.
'following' becomes 'f' (used when citing page numbers etc).
'when running' indicates anagramming the letters.
'fed'+'up'+'f'='fedupf'
'fedupf' with letters rearranged gives 'PUFFED'.
(Other definitions for puffed that I've seen before include "Out of breath" , "Blew in short gusts" , "Wheezed, being swollen" , "Breathed hard from exertion, panted" , "Panted, being breathless" .)