Lie about our first son being stringy (7)
I believe the answer is:
fibrous
'stringy' is the definition.
(I've seen this before)
'lie about our first son' is the wordplay.
'lie' becomes 'fib' (fib is a kind of lie).
'about' indicates an anagram.
'first son' becomes 's' (1st letter of 'son').
'our' is an anagram of 'rou'.
'fib'+'rou'+'s'='FIBROUS'
'being' is the link.
(Other definitions for fibrous that I've seen before include "Offering lots of roughage" , "Like roughage" , "Of, or containing, filaments" , "Stringy" .)