Kind of equation involving 50 + 1? Not far off (6)
I believe the answer is:
linear
'kind of equation' is the definition.
(I've seen this in another clue)
'50 1? not far off' is the wordplay.
'50' becomes 'l' (Roman numeral).
'1?' becomes 'I' (Roman numeral).
'not far off' becomes 'near' (I've seen this in another clue).
'l'+'i'+'near'='LINEAR'
'involving' is the link.
(Other definitions for linear that I've seen before include "-- Motor; -- equation" , "One-dimensional" , "Consisting of lines" , "adopting common narrative style" , "Nailer (anag.)" .)