One eating little head of lettuce held by cook in hot water? (7)
I believe the answer is:
slimmer
'one eating little head of lettuce' is the definition.
The definition suggests a singular noun but the answer is not.
'held by cook in hot water?' is the wordplay.
'held by' indicates putting letters inside.
'cook' becomes 'simmer' (I've seen this before).
'in hot water?' becomes 'l' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'simmer' enclosing 'l' is 'SLIMMER'.
(Other definitions for slimmer that I've seen before include "More slender" , "with less fat" , "One attempting to lose weight" , "thinner" , "I aim to get rid of some" .)