Lie about moral strength (5)
I believe the answer is:
fibre
'moral strength' is the definition.
'fibre' can be an answer for 'strength' (I have seen 'Strength of character ' mean 'fibre' so perhaps 'strength' could also mean 'fibre'). I am unsure of the 'moral' bit.
'lie about' is the wordplay.
'lie' becomes 'fib' (fibbing is a kind of liing).
'about' becomes 're' (common abbreviation for 'with reference to').
'fib'+'re'='FIBRE'
Can you help me to learn more?
(Other definitions for fibre that I've seen before include "Thread - strength of character" , "Thread, strand" , "Coarse plant food, low in nutrients" , "Dietary material; thread" , "Threadlike substance" .)