Small defence under pressure, promise to fight? (7)
I believe the answer is:
pillbox
'small defence' is the definition.
(type of small fort)
'under pressure promise to fight?' is the wordplay.
'under pressure promise' becomes 'pill' (I can't justify this - if you can you should give a lot more credence to this answer).
'to fight?' becomes 'box' (boxing is a kind of fighting).
'pill'+'box'='PILLBOX'
Can you help me to learn more?
(Other definitions for pillbox that I've seen before include "Small case for tablets - small concrete fort" , "Woman's round hat (containing tablets?)" .)