Detect medium strength chromosome (4)
I believe the answer is:
espy
'detect' is the definition.
(I've seen this before)
'medium strength chromosome' is the wordplay.
'medium strength' becomes 'esp' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'chromosome' becomes 'Y' (Y chromosome is an example).
'esp'+'y'='ESPY'
Can you help me to learn more?
(Other definitions for espy that I've seen before include "Perceive" , "Spot, notice" , "Catch sight of (poetic)" , "Make out at a distance" , "See" .)