It's not hard consuming heads of iceberg lettuce without effort? (6)
I believe the answer is:
easily
'without effort?' is the definition.
(synonyms)
'it's not hard consuming heads of iceberg lettuce' is the wordplay.
'it's not hard' becomes 'easy' ('easy' is the opposite to 'hard').
'consuming' indicates putting letters inside.
'heads of' suggests taking the first letters.
The first letters of 'iceberg lettuce' is 'il'.
'easy' placed around 'il' is 'EASILY'.
(Other definitions for easily that I've seen before include "Very probably" , "Without much difficulty" , "no problem!" , "Effortlessly" , "Simply" .)