Finished 7 day course of doxycycline for chlamydia, still have a little clear discharge which is slightly thicker now but less (male). why is this?

Sit tight for now: It can take 10+ days for urethral discharge clear entirely. If still present at 2 weeks, it could be a dual infection, i.e. chlamydia plus another unidentified cause for nonspecific urethritis (NSU), e.g. M genitalium or trichomonas. If it's still there in a few more days, recheck with the doctor or clinic treating you, and/or consider consulting with your local NHS GUM clinic. No sex until then.