Peter out in allotment finds incline (8)
I believe the answer is:
gradient
'incline' is the definition.
(I know that incline can be written as gradient)
'peter out in allotment' is the wordplay.
'peter out' becomes 'die' (I've seen this in another clue).
'in' means one lot of letters goes inside another.
'allotment' becomes 'grant' (grant is a kind of allotment).
'die' placed within 'grant' is 'GRADIENT'.
'finds' is the link.
(Other definitions for gradient that I've seen before include "Angle of slope" , "Rise" , "Measure of slope" , "Treading (anag) - slope" , "Slope or incline" .)