To confuse a lemming could be heartless! (6)
I believe the answer is:
mingle
'to confuse a lemming' is the definition.
I can't judge whether this defines the answer.
'could be heartless' is the wordplay.
'could' becomes 'ming' (I can't justify this - if you can you should believe this answer much more).
'be' becomes 'lie' (lying is a kind of being).
'heartless' means to remove the middle letters.
'lie' with its middle taken out is 'le'.
'ming'+'le'='MINGLE'
Can you help me to learn more?
(Other definitions for mingle that I've seen before include "Blend" , "Mix among others" , "Join the crowd, mix in" , "Mix (socially)" , "Circulate, socialise" .)