One small missile rising by fighter in formation (6)
I believe the answer is:
massif
'formation' is the definition.
(massif is a kind of formation)
'one small missile rising by fighter' is the wordplay.
'one' becomes 'i' (Roman numeral).
'small' becomes 's' (abbreviation - e.g. clothes size).
'missile' becomes 'SAM' (abbreviation for surface-to-air missile).
'rising' says the letters should be written in reverse (in a down clue, letters go up).
'by fighter' becomes 'f' (abbreviation in war plane e.g. F-16 etc.. I am not sure about the 'by' bit.).
'i'+'s'+'sam'='issam'
'issam' in reverse letter order is 'massi'.
'massi'+'f'='MASSIF'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for massif that I've seen before include "Compact grouping of mountains" , "Compact group of mountains" , "Mountainous block" , "Extensive raised area" , "Highland area?" .)