Feel and be strangely weak (6)
I believe the answer is:
Here is my best explanation:
'weak' is the definition.
('feeble' can be a synonym of 'weak')
'feel and be strangely' is the subsidiary indication.
'and' says to put letters next to each other (I've seen this in other clues).
'strangely' indicates an anagram.
'feel' put after 'be' is 'befeel'.
'befeel' anagrammed gives 'feeble'.
(Other definitions for feeble that I've seen before include "Weak or ineffective", "Wimpish", "lacking resolve", "Physically weak or lacking effectiveness", "Ineffective", "Frail", "rather weak", "Lacking force", "Weak from illness or age" and "Weak - unconvincing".)