Base of allotment isn't soil (5)
I believe the answer is:
taint
'soil' is the definition.
(I know that taint is a more specific form of the action contaminate)
'base of allotment isn't' is the wordplay.
'base of' suggests the final letters.
'isn't' becomes 'aint' (non-standard version of 'isn't').
The last letter of 'allotment' is 't'.
't'+'aint'='TAINT'
(Other definitions for taint that I've seen before include "Contaminate, pollute" , "Defect - contamination" , "Stigmatise" , "Sully, stain" , "Rot, spoil, define" .)