Perhaps limpet mine finally smothered by military unit before capture (3,5)
I believe the answer is:
sea snail
'perhaps limpet' is the definition.
(I know that Limpet is a type of sea snail)
'mine finally smothered by military unit before capture' is the wordplay.
'finally' indicates one should take the final letters.
'smothered by' means one lot of letters goes inside another.
'military unit' becomes 'SAS' (Special Air Service in UK army).
'before' says to put letters next to each other.
'capture' becomes 'nail' (I've seen this in another clue).
The final letter of 'mine' is 'e'.
'e' going inside 'sas' is 'seas'.
'seas'+'nail'='SEA-SNAIL'
(Other definitions for sea snail that I've seen before include "Creeping marine creature with a spiral shell" , "Spiral-shelled marine mollusc" , "wet crawler" .)