Engineer's company first had part of reactor (4)
I believe the answer is:
core
'part of reactor' is the definition.
('core' is part of 'reactor')
'engineer's company first' is the wordplay.
'engineer' becomes 're' (Royal Engineer**).
'company' becomes 'co' (abbreviation).
'first' says to put letters next to each other.
're' put after 'co' is 'CORE'.
'had' is the link.
(Other definitions for core that I've seen before include "Inmost part" , "essential element" , "Drilled rock sample" , "Crux" , "Central part of an apple" .)