Cautious to let in leaders of the enemy, being weak (6)
I believe the answer is:
watery
'weak' is the definition.
(I know that weak can be written as watery)
'cautious to let in leaders of the enemy' is the wordplay.
'cautious' becomes 'wary' (similar in meaning).
'to let in' means one lot of letters goes inside another.
'leaders of' says to take the initial letters.
The first letters of 'the enemy' is 'te'.
'wary' going around 'te' is 'WATERY'.
'being' acts as a link.
(Other definitions for watery that I've seen before include "Dilute; weak, pale" , "Overly diluted, thin" , "Insipid" , "Aqueous" , "(Of food) thin, tasteless" .)