In maturity, engineers see eye to eye (5)
I believe the answer is:
agree
'see eye to eye' is the definition.
(I've seen this before)
'in maturity engineers' is the wordplay.
'in' indicates putting letters inside.
'maturity' becomes 'age' (I've seen this before).
'engineers' becomes 're' (Royal Engineers).
'age' placed around 're' is 'AGREE'.
(Other definitions for agree that I've seen before include "Consent required" , "Settle" , "Go well together" , "Have the same opinion" , "Enjoy view in common" .)