Mountain range’s condition observed by scholar’s son (6)
I believe the answer is:
massif
'mountain range's' is the definition.
(I've seen this before)
'condition observed by scholar's son' is the wordplay.
'condition' becomes 'if' (as in 'a big if').
'observed by' means one lot of letters go next to another.
'scholar's' becomes 'ma's' (Master of Arts).
'son' becomes 's' (genealogical abbreviation for son).
'mas'+'s'='mass'
'if' put after 'mass' is 'MASSIF'.
(Other definitions for massif that I've seen before include "Central mountain mass" , "Highland area?" , "Mountain heights in compact group" , "Compact mountainous area" , "series of peaks" .)