The First Lady's back strain (5)
I believe the answer is:
sieve
'strain' is the definition.
(I know that strain can be written as sieve)
'the first lady's back' is the wordplay.
I cannot quite see how this works, but
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is found within the answer.
'lady' could be 'eve' (woman's name) and 'eve' is present in the answer.
A single letter 's' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sieve that I've seen before include "used by the Jumblies" , "Jumblies' vessel" , "Forgetful head is like it" , "Utensil with meshed bottom" , "Meshed kitchen utensil" .)