The Spanish moan, turning weak (6)
I believe the answer is:
feeble
'weak' is the definition.
(synonyms)
'the spanish moan turning' is the wordplay.
'the spanish' becomes 'el' ('the' in Spanish).
'moan' becomes 'beef' (both can informally mean a criticism).
'turning' says the letters should be written backwards.
'el'+'beef'='elbeef'
'elbeef' written backwards gives 'FEEBLE'.
(Other definitions for feeble that I've seen before include "Wimpish" , "Weak, frail" , "Weak, debilitated" , "rather weak" , "Very weak, without strength" .)