Complete control of admiral finally entering unfamiliar cargo area (12)
I believe the answer is:
stranglehold
'complete control' is the definition.
(having a stranglehold means having total control)
'admiral finally entering unfamiliar cargo area' is the wordplay.
'finally' says to take the final letters.
'entering' indicates putting letters inside.
'unfamiliar' becomes 'strange' (similar in meaning).
'cargo area' becomes 'hold' (synonyms).
The final letter of 'admiral' is 'l'.
'l' inserted inside 'strange' is 'strangle'.
'strangle'+'hold'='STRANGLEHOLD'
'of' is the link.
(Other definitions for stranglehold that I've seen before include "Inexorable grip" , "Total restriction" , "exclusive control?" , "tight grip!" , "Star long held (anag) - complete power over a person" .)