Immature insect in mountain flower, right inside (5)
I believe the answer is:
larva
'immature insect' is the definition.
(immature form of an insect)
'in mountain flower right inside' is the wordplay.
'in' is an insertion indicator.
'mountain flower' becomes 'lava' (lava might flow down a mountain).
'right' becomes 'moral' (similar in meaning).
'inside' means to look at the middle letters.
The central letter of 'moral' is 'r'.
'lava' going around 'r' is 'LARVA'.
(Other definitions for larva that I've seen before include "Juvenile form of insect" , "Wriggler" , "Manage" , "Early stage of creature" , "nit?" .)