Gain medal for gallantry, initially relieving castle (7)
I believe the answer is:
windsor
'castle' is the definition.
(I've seen this before)
'gain medal for gallantry initially relieving' is the wordplay.
'gain' becomes 'win' (win is a kind of gain).
'medal for gallantry' becomes 'DSO' (Distinguished Service Order).
'initially' suggests taking the first letters.
The first letter of 'relieving' is 'r'.
'win'+'dso'+'r'='WINDSOR'
(Other definitions for windsor that I've seen before include "-chair; -knot; -castle" , "Weekend residence of the British royal family" , "9 [HOUSE]" , "Thames town" , "Royal house (or castle)" .)