Automatic reaction of so-called dog biting some footballers (6)
I believe the answer is:
reflex
'automatic reaction' is the definition.
(I've seen this before)
'so-called dog biting some footballers' is the wordplay.
'so-called dog' becomes 'Rex' (common name for a dog).
'biting' is an insertion indicator (some letters bite or swallow others).
'some footballers' becomes 'EFL' (English Football League).
'rex' placed around 'efl' is 'REFLEX'.
'of' is the link.
(Other definitions for reflex that I've seen before include "Involuntary reaction to stimulus" , "Unthinking response" , "Automatic instinctive reaction" , "Responsive" , "Involuntary response" .)