Resistance units ordered to start by British warship (4)
I believe the answer is:
ohms
'resistance units' is the definition.
(ohm is the unit of electrical resistance)
'ordered to start by british warship' is the wordplay.
'to start' says to take the initial letters.
'by british warship' becomes 'hms' (I can't explain this - if you can you should give a lot more credence to this answer).
The initial letter of 'ordered' is 'o'.
'o'+'hms'='OHMS'
Can you help me to learn more?
(Other definitions for ohms that I've seen before include "Frank marking" , "working for the monarchy?" , "Letters with official status in correspondence" , "lab units" , "measures of resistance" .)