A sentry briefly hugged by second love -- it's prickly! (7)
I believe the answer is:
saguaro
'it's prickly' is the definition.
(type of cactus)
'a sentry briefly hugged by second love' is the wordplay.
'a sentry' becomes 'guard' (I've seen this before).
'briefly' means to remove the last letter.
'hugged by' indicates putting letters inside (I've seen this in other clues).
'second love' becomes 'sao' (I can't explain this - if you can you should believe this answer much more).
'guard' with its last letter removed is 'guar'.
'guar' placed within 'sao' is 'SAGUARO'.
Can you help me to learn more?
(Other definitions for saguaro that I've seen before include "Branched cactus" , "Large branching cactus" , "Huge cactus" , "Giant type of cactus" .)