Inflammable substance from an upturned tree (6)
I believe the answer is:
napalm
'inflammable substance' is the definition.
(I've seen this in another clue)
'an upturned tree' is the wordplay.
'upturned' is a reversal indicator.
'tree' becomes 'palm' (palm is a kind of tree).
'an' back-to-front is 'na'.
'na'+'palm'='NAPALM'
'from' is the link.
(Other definitions for napalm that I've seen before include "Incendiary agent" , "Highly flammable jelly" , "Flammable jelly used in weapons" , "Does Ma plan to use the sticky jelly in fire bombs?" , "Incendiary bomb ingredient" .)