Snoop around observation post being sentimental (5)
I believe the answer is:
soppy
'being sentimental' is the definition.
(I've seen this before)
'snoop around observation post' is the wordplay.
'snoop' becomes 'spy' (synonyms).
'around' means one lot of letters goes inside another.
'observation post' becomes 'op' (this could be a standard abbreviation which I don't know about).
'spy' enclosing 'op' is 'SOPPY'.
Can you help me to learn more?
(Other definitions for soppy that I've seen before include "Emotional" , "Indulgently sentimental" , "Drenched" , "Lovey-dovey" , "Mushy" .)