Family outside on marks close to royal citadel (7)
I believe the answer is:
kremlin
'royal citadel' is the definition.
'kremlin' can be an answer for 'citadel' (kremlin is a kind of citadel). I'm not sure about the 'royal' bit.
'family outside on marks close' is the wordplay.
'family' becomes 'kin' (synonyms).
'outside' means one lot of letters goes inside another (some letters go outside others).
'on' becomes 're' ('on' can be mean 'with reference to').
'marks close' becomes 'ml' (I can't justify this - if you can you should believe this answer much more).
're'+'ml'='reml'
'kin' enclosing 'reml' is 'KREMLIN'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for kremlin that I've seen before include "Moscow landmark" , "fortress" , "Symbol of the Russian government" , "powerful red element?" , "Citadel of a Russian town" .)