Small plane ultimately inadequate, in truth (5)
I believe the answer is:
facet
'small plane' is the definition.
'facet' can be an answer for 'plane' (facet plane is a kind of plane). I am unsure of the 'small' bit.
'ultimately inadequate in truth' is the wordplay.
'ultimately' suggests the final letters.
'in' indicates putting letters inside.
'truth' becomes 'fact' (I've seen this before).
The final letter of 'inadequate' is 'e'.
'e' inserted into 'fact' is 'FACET'.
Can you help me to learn more?
(Other definitions for facet that I've seen before include "side of rock" , "Side of a cut gem" , "Side or surface, diamond or building" , "Aspect or feature of something" , "Gem's cut side" .)