Pair inside saw first of lags fall headlong (6)
I believe the answer is:
sprawl
'fall headlong' is the definition.
(thesaurus)
'pair inside saw first of lags' is the wordplay.
'pair' becomes 'pr' (abbreviation).
'inside' is an insertion indicator.
'first of lags' becomes 'l' (1st letter of 'lags').
'pr' going inside 'saw' is 'spraw'.
'spraw'+'l'='SPRAWL'
(Other definitions for sprawl that I've seen before include "Lounge" , "Urban or industrial expansion" , "Sit lazily, with limbs spread wide" , "Uncontrolled urban development" , "Lie with arms and legs spread about loosely" .)