Engineers have couple to fix (6)
I believe the answer is:
repair
'fix' is the definition.
(to fix something is to repair it)
'engineers have couple' is the wordplay.
I cannot quite see how this works, but
'engineers' could be 're' (abbreviation for Royal Engineers) and 're' is located in the answer.
'couple' could be 'pair' (synonyms) and 'pair' is present in the answer.
No letters remain.
This explanation may well be incorrect...
'to' is the link.
Can you help me to learn more?
(Other definitions for repair that I've seen before include "Make one's way (to); fix" , "Travel or proceed toward some place" , "retire" , "Fix, mend" , "It may be effected by" .)