American daughter is lacking in muscle tone (4)
I believe the answer is:
aqua
'lacking in muscle tone' is the definition.
The definition and answer can be both human attributes as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'american daughter' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'daughter' becomes 'qua' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'qua'='AQUA'
'is' is the link.
Can you help me to learn more?
(Other definitions for aqua that I've seen before include "Blue tinged with green" , "Hue" , "(Denoting) water (Lat)" , "which supports boats?" , "Water (L)" .)