Small details oddly missing in stamp (4)
I believe the answer is:
seal
'stamp' is the definition.
(synonyms)
'small details oddly missing' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'oddly missing' indicates alternate letters (letters in odd-numbered positions removed).
The alternating letters of 'details' are 'eal'.
's'+'eal'='SEAL'
'in' acts as a link.
(Other definitions for seal that I've seen before include "Amphibious fish-eating sea mammal" , "Fasten with hot wax" , "Imprinted disk affixed to a document" , "relative of the walrus" , "Secure against leakage" .)