Moan about feeble National Trust (6)
I believe the answer is:
lament
'moan' is the definition.
(thesaurus)
'feeble national trust' is the wordplay.
'feeble' becomes 'lame' ('lame' can be a synonym of 'feeble').
'national trust' becomes 'nt'.
'lame'+'nt'='LAMENT'
'about' acts as a link.
(Other definitions for lament that I've seen before include "Keen" , "Elegy" , "Express disappointment" , "Show of sorrow" , "Sad song" .)